mergetool(vimdiff): allow paths to contain spaces again

In 0041797449 (vimdiff: new implementation with layout support,
2022-03-30), we introduced a completely new implementation of the
`vimdiff` backend for `git mergetool`.

In this implementation, we no longer call `vim` directly but we
accumulate in the variable `FINAL_CMD` an arbitrary number of commands
for `vim` to execute, which necessitates the use of `eval` to split the
commands properly into multiple command-line arguments.

That same `eval` command also needs to pass the paths to `vim`, and
while it looks as if they are quoted correctly, that quoting only
reaches the `eval` instruction and is lost after that, therefore paths
that contain whitespace characters (or other characters that are
interpreted by the POSIX shell) are handled incorrectly.

This is a simple reproducer:

	git init -b main bam-merge-fail
	cd bam-merge-fail
	echo a>"a file.txt"
	git add "a file.txt"
	git commit -m "added 'a file.txt'"
	echo b>"a file.txt"
	git add "a file.txt"
	git commit -m "diverged b 'a file.txt'"
	git checkout -b c HEAD~
	echo c>"a file.txt"
	git add "a file.txt"
	git commit -m "diverged c 'a file.txt'"
	git checkout main
	git merge c
	git mergetool --tool=vimdiff

With Git v2.37.0/v2.37.1, this will open 7 buffers, not four, and not
display the correct contents at all.

To fix this, let's not expand the variables containing the path
parameters before passing them to the `eval` command, but let that
command expand the variables instead.

This fixes https://github.com/git-for-windows/git/issues/3945

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/vimdiff

@@ -414,8 +414,8 @@ merge_cmd () {
 
 	if $base_present
 	then
-		eval "$merge_tool_path" \
-			-f "$FINAL_CMD" "$LOCAL" "$BASE" "$REMOTE" "$MERGED"
+		eval '"$merge_tool_path"' \
+			-f "$FINAL_CMD" '"$LOCAL"' '"$BASE"' '"$REMOTE"' '"$MERGED"'
 	else
 		# If there is no BASE (example: a merge conflict in a new file
 		# with the same name created in both braches which didn't exist
@@ -424,8 +424,8 @@ merge_cmd () {
 		FINAL_CMD=$(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" "$MERGED"
+		eval '"$merge_tool_path"' \
+			-f "$FINAL_CMD" '"$LOCAL"' '"$REMOTE"' '"$MERGED"'
 	fi
 
 	ret="$?"
@@ -614,6 +614,37 @@ run_unit_tests () {
 		fi
 	done
 
+	# verify that `merge_cmd` handles paths with spaces
+	record_parameters () {
+		>actual
+		for arg
+		do
+			echo "$arg" >>actual
+		done
+	}
+
+	base_present=false
+	LOCAL='lo cal'
+	BASE='ba se'
+	REMOTE="' '"
+	MERGED='mer ged'
+	merge_tool_path=record_parameters
+
+	merge_cmd vimdiff || at_least_one_ko=true
+
+	cat >expect <<-\EOF
+	-f
+	-c
+	echo | split | vertical split | 1b | wincmd l | vertical split | quit | wincmd l | 2b | wincmd j | 3b | tabdo windo diffthis
+	-c
+	tabfirst
+	lo cal
+	' '
+	mer ged
+	EOF
+
+	diff -u expect actual || at_least_one_ko=true
+
 	if test "$at_least_one_ko" = "true"
 	then
 		return 255