difftool: Allow specifying unconfigured commands with --extcmd

git-difftool requires difftool.<tool>.cmd configuration even when
tools use the standard "$diffcmd $from $to" form.  This teaches
git-difftool to run these tools in lieu of configuration by
allowing the command to be specified on the command line.

Reference: http://article.gmane.org/gmane.comp.version-control.git/133377
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-difftool.txt

@@ -58,6 +58,11 @@ is set to the name of the temporary file containing the contents
 of the diff post-image.  `$BASE` is provided for compatibility
 with custom merge tool commands and has the same value as `$LOCAL`.
 
+--extcmd=<command>::
+	Specify a custom command for viewing diffs.
+	'git-difftool' ignores the configured defaults and runs
+	`$command $LOCAL $REMOTE` when this option is specified.
+
 -g::
 --gui::
 	When 'git-difftool' is invoked with the `-g` or `--gui` option

git-difftool--helper.sh

@@ -19,6 +19,11 @@ should_prompt () {
 	fi
 }
 
+# Indicates that --extcmd=... was specified
+use_ext_cmd () {
+	test -n "$GIT_DIFFTOOL_EXTCMD"
+}
+
 launch_merge_tool () {
 	# Merged is the filename as it appears in the work tree
 	# Local is the contents of a/filename
@@ -33,18 +38,29 @@ launch_merge_tool () {
 	# the user with the real $MERGED name before launching $merge_tool.
 	if should_prompt; then
 		printf "\nViewing: '$MERGED'\n"
-		printf "Hit return to launch '%s': " "$merge_tool"
+		if use_ext_cmd; then
+			printf "Hit return to launch '%s': " \
+				"$GIT_DIFFTOOL_EXTCMD"
+		else
+			printf "Hit return to launch '%s': " "$merge_tool"
+		fi
 		read ans
 	fi
 
-	run_merge_tool "$merge_tool"
+	if use_ext_cmd; then
+		$GIT_DIFFTOOL_EXTCMD "$LOCAL" "$REMOTE"
+	else
+		run_merge_tool "$merge_tool"
+	fi
+
 }
 
-# GIT_DIFF_TOOL indicates that --tool=... was specified
-if test -n "$GIT_DIFF_TOOL"; then
-	merge_tool="$GIT_DIFF_TOOL"
-else
-	merge_tool="$(get_merge_tool)" || exit
+if ! use_ext_cmd; then
+	if test -n "$GIT_DIFF_TOOL"; then
+		merge_tool="$GIT_DIFF_TOOL"
+	else
+		merge_tool="$(get_merge_tool)" || exit
+	fi
 fi
 
 # Launch the merge tool on each path provided by 'git diff'

git-difftool.perl

@@ -62,6 +62,10 @@ sub generate_command
 			$skip_next = 1;
 			next;
 		}
+		if ($arg =~ /^--extcmd=/) {
+			$ENV{GIT_DIFFTOOL_EXTCMD} = substr($arg, 9);
+			next;
+		}
 		if ($arg =~ /^--tool=/) {
 			$ENV{GIT_DIFF_TOOL} = substr($arg, 7);
 			next;

t/t7800-difftool.sh

@@ -214,7 +214,24 @@ test_expect_success 'difftool.<tool>.path' '
 	diff=$(git difftool --tool=tkdiff --no-prompt branch) &&
 	git config --unset difftool.tkdiff.path &&
 	lines=$(echo "$diff" | grep file | wc -l) &&
-	test "$lines" -eq 1
+	test "$lines" -eq 1 &&
+
+	restore_test_defaults
+'
+
+test_expect_success 'difftool --extcmd=...' '
+	diff=$(git difftool --no-prompt --extcmd=cat branch) &&
+
+	lines=$(echo "$diff" | wc -l) &&
+	test "$lines" -eq 2 &&
+
+	lines=$(echo "$diff" | grep master | wc -l) &&
+	test "$lines" -eq 1 &&
+
+	lines=$(echo "$diff" | grep branch | wc -l) &&
+	test "$lines" -eq 1 &&
+
+	restore_test_defaults
 '
 
 test_done