diff --git a/mergetools/vimdiff b/mergetools/vimdiff
index 96f6209a04..9d1bf4f455 100644
--- a/mergetools/vimdiff
+++ b/mergetools/vimdiff
@@ -1,49 +1,442 @@
+# This script can be run in two different contexts:
+#
+#   - From git, when the user invokes the "vimdiff" merge tool. In this context
+#     this script expects the following environment variables (among others) to
+#     be defined (which is something "git" takes care of):
+#
+#       - $BASE
+#       - $LOCAL
+#       - $REMOTE
+#       - $MERGED
+#
+#     In this mode, all this script does is to run the next command:
+#
+#         vim -f -c ... $LOCAL $BASE $REMOTE $MERGED
+#
+#     ...where the "..." string depends on the value of the
+#     "mergetool.vimdiff.layout" configuration variable and is used to open vim
+#     with a certain layout of buffers, windows and tabs.
+#
+#   - From a script inside the unit tests framework folder ("t" folder) by
+#     sourcing this script and then manually calling "run_unit_tests", which
+#     will run a battery of unit tests to make sure nothing breaks.
+#     In this context this script does not expect any particular environment
+#     variable to be set.
+
+
+################################################################################
+## Internal functions (not meant to be used outside this script)
+################################################################################
+
+debug_print () {
+	# Send message to stderr if global variable GIT_MERGETOOL_VIMDIFF is set
+	# to "true"
+
+	if test -n "$GIT_MERGETOOL_VIMDIFF_DEBUG"
+	then
+		>&2 echo "$@"
+	fi
+}
+
+substring () {
+	# Return a substring of $1 containing $3 characters starting at
+	# zero-based offset $2.
+	#
+	# Examples:
+	#
+	#   substring "Hello world" 0 4  --> "Hell"
+	#   substring "Hello world" 3 4  --> "lo w"
+	#   substring "Hello world" 3 10 --> "lo world"
+
+	STRING=$1
+	START=$2
+	LEN=$3
+
+	echo "$STRING" | cut -c$(( START + 1 ))-$(( START + $LEN ))
+}
+
+gen_cmd_aux () {
+	# Auxiliary function used from "gen_cmd()".
+	# Read that other function documentation for more details.
+
+	LAYOUT=$1
+	CMD=$2  # This is a second (hidden) argument used for recursion
+
+	debug_print
+	debug_print "LAYOUT    : $LAYOUT"
+	debug_print "CMD       : $CMD"
+
+	if test -z "$CMD"
+	then
+		CMD="echo" # vim "nop" operator
+	fi
+
+	start=0
+	end=${#LAYOUT}
+
+	nested=0
+	nested_min=100
+
+
+	# Step 1:
+	#
+	# Increase/decrease "start"/"end" indices respectively to get rid of
+	# outer parenthesis.
+	#
+	# Example:
+	#
+	#   - BEFORE: (( LOCAL , BASE ) / MERGED )
+	#   - AFTER :  ( LOCAL , BASE ) / MERGED
+
+	oldIFS=$IFS
+	IFS=#
+	for c in $(echo "$LAYOUT" | sed 's:.:&#:g')
+	do
+		if test "$c" = " "
+		then
+			continue
+		fi
+
+		if test "$c" = "("
+		then
+			nested=$(( nested + 1 ))
+			continue
+		fi
+
+		if test "$c" = ")"
+		then
+			nested=$(( nested - 1 ))
+			continue
+		fi
+
+		if test "$nested" -lt "$nested_min"
+		then
+			nested_min=$nested
+		fi
+	done
+	IFS=$oldIFS
+
+	debug_print "NESTED MIN: $nested_min"
+
+	while test "$nested_min" -gt "0"
+	do
+		start=$(( start + 1 ))
+		end=$(( end - 1 ))
+
+		start_minus_one=$(( start - 1 ))
+
+		while ! test "$(substring "$LAYOUT" "$start_minus_one" 1)" = "("
+		do
+			start=$(( start + 1 ))
+			start_minus_one=$(( start_minus_one + 1 ))
+		done
+
+		while ! test "$(substring "$LAYOUT" "$end" 1)" = ")"
+		do
+			end=$(( end - 1 ))
+		done
+
+		nested_min=$(( nested_min - 1 ))
+	done
+
+	debug_print "CLEAN     : $(substring "$LAYOUT" "$start" "$(( end - start ))")"
+
+
+	# Step 2:
+	#
+	# Search for all valid separators ("+", "/" or ",") which are *not*
+	# inside parenthesis. Save the index at which each of them makes the
+	# first appearance.
+
+	index_new_tab=""
+	index_horizontal_split=""
+	index_vertical_split=""
+
+	nested=0
+	i=$(( start - 1 ))
+
+	oldIFS=$IFS
+	IFS=#
+	for c in $(substring "$LAYOUT" "$start" "$(( end - start ))" | sed 's:.:&#:g');
+	do
+		i=$(( i + 1 ))
+
+		if test "$c" = " "
+		then
+			continue
+		fi
+
+		if test "$c" = "("
+		then
+			nested=$(( nested + 1 ))
+			continue
+		fi
+
+		if test "$c" = ")"
+		then
+			nested=$(( nested - 1 ))
+			continue
+		fi
+
+		if test "$nested" = 0
+		then
+			current=$c
+
+			if test "$current" = "+"
+			then
+				if test -z "$index_new_tab"
+				then
+					index_new_tab=$i
+				fi
+
+			elif test "$current" = "/"
+			then
+				if test -z "$index_horizontal_split"
+				then
+					index_horizontal_split=$i
+				fi
+
+			elif test "$current" = ","
+			then
+				if test -z "$index_vertical_split"
+				then
+					index_vertical_split=$i
+				fi
+			fi
+		fi
+	done
+	IFS=$oldIFS
+
+
+	# Step 3:
+	#
+	# Process the separator with the highest order of precedence
+	# (";" has the highest precedence and "|" the lowest one).
+	#
+	# By "process" I mean recursively call this function twice: the first
+	# one with the substring at the left of the separator and the second one
+	# with the one at its right.
+
+	terminate="false"
+
+	if ! test -z "$index_new_tab"
+	then
+		before="-tabnew"
+		after="tabnext"
+		index=$index_new_tab
+		terminate="true"
+
+	elif ! test -z "$index_horizontal_split"
+	then
+		before="split"
+		after="wincmd j"
+		index=$index_horizontal_split
+		terminate="true"
+
+	elif ! test -z "$index_vertical_split"
+	then
+		before="vertical split"
+		after="wincmd l"
+		index=$index_vertical_split
+		terminate="true"
+	fi
+
+	if  test "$terminate" = "true"
+	then
+		CMD="$CMD | $before"
+		CMD=$(gen_cmd_aux "$(substring "$LAYOUT" "$start" "$(( index - start ))")" "$CMD")
+		CMD="$CMD | $after"
+		CMD=$(gen_cmd_aux "$(substring "$LAYOUT" "$(( index + 1 ))" "$(( ${#LAYOUT} - index ))")" "$CMD")
+		echo "$CMD"
+		return
+	fi
+
+
+	# Step 4:
+	#
+	# If we reach this point, it means there are no separators and we just
+	# need to print the command to display the specified buffer
+
+	target=$(substring "$LAYOUT" "$start" "$(( end - start ))" | sed 's:[ @();|-]::g')
+
+	if test "$target" = "LOCAL"
+	then
+		CMD="$CMD | 1b"
+
+	elif test "$target" = "BASE"
+	then
+		CMD="$CMD | 2b"
+
+	elif test "$target" = "REMOTE"
+	then
+		CMD="$CMD | 3b"
+
+	elif test "$target" = "MERGED"
+	then
+		CMD="$CMD | 4b"
+
+	else
+		CMD="$CMD | ERROR: >$target<"
+	fi
+
+	echo "$CMD"
+	return
+}
+
+
+gen_cmd () {
+	# This function returns (in global variable FINAL_CMD) the string that
+	# you can use when invoking "vim" (as shown next) to obtain a given
+	# layout:
+	#
+	#   $ vim -f $FINAL_CMD "$LOCAL" "$BASE" "$REMOTE" "$MERGED"
+	#
+	# It takes one single argument: a string containing the desired layout
+	# definition.
+	#
+	# The syntax of the "layout definitions" is explained in "Documentation/
+	# mergetools/vimdiff.txt" but you can already intuitively understand how
+	# it works by knowing that...
+	#
+	#   * "+" means "a new vim tab"
+	#   * "/" means "a new vim horizontal split"
+	#   * "," means "a new vim vertical split"
+	#
+	# It also returns (in global variable FINAL_TARGET) the name ("LOCAL",
+	# "BASE", "REMOTE" or "MERGED") of the file that is marked with an "@",
+	# or "MERGED" if none of them is.
+	#
+	# Example:
+	#
+	#     gen_cmd "@LOCAL , REMOTE"
+	#     |
+	#     `-> FINAL_CMD    == "-c \"echo | vertical split | 1b | wincmd l | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	#         FINAL_TARGET == "LOCAL"
+
+	LAYOUT=$1
+
+
+	# Search for a "@" in one of the files identifiers ("LOCAL", "BASE",
+	# "REMOTE", "MERGED"). If not found, use "MERGE" as the default file
+	# where changes will be saved.
+
+	if echo "$LAYOUT" | grep @LOCAL >/dev/null
+	then
+		FINAL_TARGET="LOCAL"
+	elif echo "$LAYOUT" | grep @BASE >/dev/null
+	then
+		FINAL_TARGET="BASE"
+	else
+		FINAL_TARGET="MERGED"
+	fi
+
+
+	# Obtain the first part of vim "-c" option to obtain the desired layout
+
+	CMD=$(gen_cmd_aux "$LAYOUT")
+
+
+	# Adjust the just obtained script depending on whether more than one
+	# windows are visible or not
+
+	if echo "$LAYOUT" | grep ",\|/" >/dev/null
+	then
+		CMD="$CMD | tabdo windo diffthis"
+	else
+		CMD="$CMD | bufdo diffthis"
+	fi
+
+
+	# Add an extra "-c" option to move to the first tab (notice that we
+	# can't simply append the command to the previous "-c" string as
+	# explained here: https://github.com/vim/vim/issues/9076
+
+	FINAL_CMD="-c \"$CMD\" -c \"tabfirst\""
+}
+
+
+################################################################################
+## API functions (called from "git-mergetool--lib.sh")
+################################################################################
+
 diff_cmd () {
 	"$merge_tool_path" -R -f -d \
 		-c 'wincmd l' -c 'cd $GIT_PREFIX' "$LOCAL" "$REMOTE"
 }
 
+
 merge_cmd () {
+	layout=$(git config mergetool.vimdiff.layout)
+
 	case "$1" in
 	*vimdiff)
-		if $base_present
+		if test -z "$layout"
 		then
-			"$merge_tool_path" -f -d -c '4wincmd w | wincmd J' \
-				"$LOCAL" "$BASE" "$REMOTE" "$MERGED"
-		else
-			"$merge_tool_path" -f -d -c 'wincmd l' \
-				"$LOCAL" "$MERGED" "$REMOTE"
+			# Default layout when none is specified
+			layout="(LOCAL,BASE,REMOTE)/MERGED"
 		fi
 		;;
 	*vimdiff1)
-		"$merge_tool_path" -f -d \
-			-c 'echon "Resolve conflicts leftward then save. Use :cq to abort."' \
-			"$LOCAL" "$REMOTE"
-		ret="$?"
-		if test "$ret" -eq 0
-		then
-			cp -- "$LOCAL" "$MERGED"
-		fi
-		return "$ret"
+		layout="@LOCAL,REMOTE"
 		;;
 	*vimdiff2)
-		"$merge_tool_path" -f -d -c 'wincmd l' \
-			"$LOCAL" "$MERGED" "$REMOTE"
+		layout="LOCAL,MERGED,REMOTE"
 		;;
 	*vimdiff3)
-		if $base_present
-		then
-			"$merge_tool_path" -f -d -c 'hid | hid | hid' \
-				"$LOCAL" "$REMOTE" "$BASE" "$MERGED"
-		else
-			"$merge_tool_path" -f -d -c 'hid | hid' \
-				"$LOCAL" "$REMOTE" "$MERGED"
-		fi
+		layout="MERGED"
 		;;
 	esac
+
+	gen_cmd "$layout"
+
+	debug_print ""
+	debug_print "FINAL CMD : $FINAL_CMD"
+	debug_print "FINAL TAR : $FINAL_TARGET"
+
+	if $base_present
+	then
+		eval "$merge_tool_path" \
+			-f "$FINAL_CMD" "$LOCAL" "$BASE" "$REMOTE" "$MERGED"
+	else
+		# If there is no BASE (example: a merge conflict in a new file
+		# with the same name created in both braches which didn't exist
+		# before), close all BASE windows using vim's "quit" command
+
+		FINAL_CMD=$(echo "$FINAL_CMD" | \
+			sed -e 's:2b:quit:g' -e 's:3b:2b:g' -e 's:4b:3b:g')
+
+		eval "$merge_tool_path" \
+			-f "$FINAL_CMD" "$LOCAL" "$REMOTE" "$MERGED"
+	fi
+
+	ret="$?"
+
+	if test "$ret" -eq 0
+	then
+		case "$FINAL_TARGET" in
+		LOCAL)
+			source_path="$LOCAL"
+			;;
+		REMOTE)
+			source_path="$REMOTE"
+			;;
+		MERGED|*)
+			# Do nothing
+			source_path=
+			;;
+		esac
+
+		if test -n "$source_path"
+		then
+			cp "$source_path" "$MERGED"
+		fi
+	fi
+
+	return "$ret"
 }
 
-translate_merge_tool_path() {
+
+translate_merge_tool_path () {
 	case "$1" in
 	nvimdiff*)
 		echo nvim
@@ -57,14 +450,121 @@ translate_merge_tool_path() {
 	esac
 }
 
+
 exit_code_trustable () {
 	true
 }
 
+
 list_tool_variants () {
-	for prefix in '' g n; do
-		for suffix in '' 1 2 3; do
-			echo "${prefix}vimdiff${suffix}"
+	if test "$TOOL_MODE" = "diff"
+	then
+		for prefix in '' g n
+		do
+			echo "${prefix}vimdiff"
 		done
-	done
+	else
+		for prefix in '' g n
+		do
+			for suffix in '' 1 2 3
+			do
+				echo "${prefix}vimdiff${suffix}"
+			done
+		done
+	fi
+}
+
+
+################################################################################
+## Unit tests (called from scripts inside the "t" folder)
+################################################################################
+
+run_unit_tests () {
+	# Function to make sure that we don't break anything when modifying this
+	# script.
+
+	NUMBER_OF_TEST_CASES=16
+
+	TEST_CASE_01="(LOCAL,BASE,REMOTE)/MERGED"   # default behaviour
+	TEST_CASE_02="@LOCAL,REMOTE"                # when using vimdiff1
+	TEST_CASE_03="LOCAL,MERGED,REMOTE"          # when using vimdiff2
+	TEST_CASE_04="MERGED"                       # when using vimdiff3
+	TEST_CASE_05="LOCAL/MERGED/REMOTE"
+	TEST_CASE_06="(LOCAL/REMOTE),MERGED"
+	TEST_CASE_07="MERGED,(LOCAL/REMOTE)"
+	TEST_CASE_08="(LOCAL,REMOTE)/MERGED"
+	TEST_CASE_09="MERGED/(LOCAL,REMOTE)"
+	TEST_CASE_10="(LOCAL/BASE/REMOTE),MERGED"
+	TEST_CASE_11="(LOCAL,BASE,REMOTE)/MERGED+BASE,LOCAL+BASE,REMOTE+(LOCAL/BASE/REMOTE),MERGED"
+	TEST_CASE_12="((LOCAL,REMOTE)/BASE),MERGED"
+	TEST_CASE_13="((LOCAL,REMOTE)/BASE),((LOCAL/REMOTE),MERGED)"
+	TEST_CASE_14="BASE,REMOTE+BASE,LOCAL"
+	TEST_CASE_15="  ((  (LOCAL , BASE , REMOTE) / MERGED))   +(BASE)   , LOCAL+ BASE , REMOTE+ (((LOCAL / BASE / REMOTE)) ,    MERGED   )  "
+	TEST_CASE_16="LOCAL,BASE,REMOTE / MERGED + BASE,LOCAL + BASE,REMOTE + (LOCAL / BASE / REMOTE),MERGED"
+
+	EXPECTED_CMD_01="-c \"echo | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_02="-c \"echo | vertical split | 1b | wincmd l | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_03="-c \"echo | vertical split | 1b | wincmd l | vertical split | 4b | wincmd l | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_04="-c \"echo | 4b | bufdo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_05="-c \"echo | split | 1b | wincmd j | split | 4b | wincmd j | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_06="-c \"echo | vertical split | split | 1b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_07="-c \"echo | vertical split | 4b | wincmd l | split | 1b | wincmd j | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_08="-c \"echo | split | vertical split | 1b | wincmd l | 3b | wincmd j | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_09="-c \"echo | split | 4b | wincmd j | vertical split | 1b | wincmd l | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_10="-c \"echo | vertical split | split | 1b | wincmd j | split | 2b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_11="-c \"echo | -tabnew | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabnext | -tabnew | vertical split | 2b | wincmd l | 1b | tabnext | -tabnew | vertical split | 2b | wincmd l | 3b | tabnext | vertical split | split | 1b | wincmd j | split | 2b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_12="-c \"echo | vertical split | split | vertical split | 1b | wincmd l | 3b | wincmd j | 2b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_13="-c \"echo | vertical split | split | vertical split | 1b | wincmd l | 3b | wincmd j | 2b | wincmd l | vertical split | split | 1b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_14="-c \"echo | -tabnew | vertical split | 2b | wincmd l | 3b | tabnext | vertical split | 2b | wincmd l | 1b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_15="-c \"echo | -tabnew | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabnext | -tabnew | vertical split | 2b | wincmd l | 1b | tabnext | -tabnew | vertical split | 2b | wincmd l | 3b | tabnext | vertical split | split | 1b | wincmd j | split | 2b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_16="-c \"echo | -tabnew | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabnext | -tabnew | vertical split | 2b | wincmd l | 1b | tabnext | -tabnew | vertical split | 2b | wincmd l | 3b | tabnext | vertical split | split | 1b | wincmd j | split | 2b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+
+	EXPECTED_TARGET_01="MERGED"
+	EXPECTED_TARGET_02="LOCAL"
+	EXPECTED_TARGET_03="MERGED"
+	EXPECTED_TARGET_04="MERGED"
+	EXPECTED_TARGET_05="MERGED"
+	EXPECTED_TARGET_06="MERGED"
+	EXPECTED_TARGET_07="MERGED"
+	EXPECTED_TARGET_08="MERGED"
+	EXPECTED_TARGET_09="MERGED"
+	EXPECTED_TARGET_10="MERGED"
+	EXPECTED_TARGET_11="MERGED"
+	EXPECTED_TARGET_12="MERGED"
+	EXPECTED_TARGET_13="MERGED"
+	EXPECTED_TARGET_14="MERGED"
+	EXPECTED_TARGET_15="MERGED"
+	EXPECTED_TARGET_16="MERGED"
+
+	at_least_one_ko="false"
+
+	for i in $(seq -w 1 99)
+	do
+		if test "$i" -gt $NUMBER_OF_TEST_CASES
+		then
+			break
+		fi
+
+		gen_cmd "$(eval echo \${TEST_CASE_"$i"})"
+
+		if test "$FINAL_CMD" = "$(eval echo \${EXPECTED_CMD_"$i"})" \
+			&& test "$FINAL_TARGET" = "$(eval echo \${EXPECTED_TARGET_"$i"})"
+		then
+			printf "Test Case #%02d: OK\n" "$(echo "$i" | sed 's/^0*//')"
+		else
+			printf "Test Case #%02d: KO !!!!\n" "$(echo "$i" | sed 's/^0*//')"
+			echo "  FINAL_CMD              : $FINAL_CMD"
+			echo "  FINAL_CMD (expected)   : $(eval echo \${EXPECTED_CMD_"$i"})"
+			echo "  FINAL_TARGET           : $FINAL_TARGET"
+			echo "  FINAL_TARGET (expected): $(eval echo \${EXPECTED_TARGET_"$i"})"
+			at_least_one_ko="true"
+		fi
+	done
+
+	if test "$at_least_one_ko" = "true"
+	then
+		return 255
+	else
+		return 0
+	fi
 }