git-mergetool--lib: Make vimdiff retain the current directory

When using difftool with vimdiff it can be unexpected that
the current directory changes to the root of the project.
Tell vim to chdir to the value of $GIT_PREFIX to fix this.

Care is taken to quote the variable so that vim expands it.
This avoids problems when directory names contain spaces.

Signed-off-by: David Aguilar <davvid@gmail.com>
Reported-by: Frédéric Heitzmann <frederic.heitzmann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -86,6 +86,11 @@ get_merge_tool_cmd () {
 }
 
 run_merge_tool () {
+	# If GIT_PREFIX is empty then we cannot use it in tools
+	# that expect to be able to chdir() to its value.
+	GIT_PREFIX=${GIT_PREFIX:-.}
+	export GIT_PREFIX
+
 	merge_tool_path="$(get_merge_tool_path "$1")" || exit
 	base_present="$2"
 	status=0
@@ -188,6 +193,7 @@ run_merge_tool () {
 			check_unchanged
 		else
 			"$merge_tool_path" -R -f -d -c "wincmd l" \
+				-c 'cd $GIT_PREFIX' \
 				"$LOCAL" "$REMOTE"
 		fi
 		;;
@@ -199,6 +205,7 @@ run_merge_tool () {
 			check_unchanged
 		else
 			"$merge_tool_path" -R -f -d -c "wincmd l" \
+				-c 'cd $GIT_PREFIX' \
 				"$LOCAL" "$REMOTE"
 		fi
 		;;