mergetool: add an option for writing to a temporary directory

Teach mergetool to write files in a temporary directory when
'mergetool.writeToTemp' is true.

This is helpful for tools such as Eclipse which cannot cope with
multiple copies of the same file in the worktree.

Suggested-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1755,6 +1755,12 @@ mergetool.keepTemporaries::
 	preserved, otherwise they will be removed after the tool has
 	exited. Defaults to `false`.
 
+mergetool.writeToTemp::
+	Git writes temporary 'BASE', 'LOCAL', and 'REMOTE' versions of
+	conflicting files in the worktree by default.  Git will attempt
+	to use a temporary directory for these files when set `true`.
+	Defaults to `false`.
+
 mergetool.prompt::
 	Prompt before each invocation of the merge resolution program.
 

git-mergetool.sh

@@ -37,6 +37,19 @@ base_present () {
 	test -n "$base_mode"
 }
 
+mergetool_tmpdir_init () {
+	if test "$(git config --bool mergetool.writeToTemp)" != true
+	then
+		MERGETOOL_TMPDIR=.
+		return 0
+	fi
+	if MERGETOOL_TMPDIR=$(mktemp -d -t "git-mergetool-XXXXXX" 2>/dev/null)
+	then
+		return 0
+	fi
+	die "error: mktemp is needed when 'mergetool.writeToTemp' is true"
+}
+
 cleanup_temp_files () {
 	if test "$1" = --save-backup
 	then
@@ -46,6 +59,10 @@ cleanup_temp_files () {
 	else
 		rm -f -- "$LOCAL" "$REMOTE" "$BASE" "$BACKUP"
 	fi
+	if test "$MERGETOOL_TMPDIR" != "."
+	then
+		rmdir "$MERGETOOL_TMPDIR"
+	fi
 }
 
 describe_file () {
@@ -235,10 +252,20 @@ merge_file () {
 		BASE=$MERGED
 		ext=
 	fi
-	BACKUP="./${BASE}_BACKUP_$$$ext"
-	LOCAL="./${BASE}_LOCAL_$$$ext"
-	REMOTE="./${BASE}_REMOTE_$$$ext"
-	BASE="./${BASE}_BASE_$$$ext"
+
+	mergetool_tmpdir_init
+
+	if test "$MERGETOOL_TMPDIR" != "."
+	then
+		# If we're using a temporary directory then write to the
+		# top-level of that directory.
+		BASE=${BASE##*/}
+	fi
+
+	BACKUP="$MERGETOOL_TMPDIR/${BASE}_BACKUP_$$$ext"
+	LOCAL="$MERGETOOL_TMPDIR/${BASE}_LOCAL_$$$ext"
+	REMOTE="$MERGETOOL_TMPDIR/${BASE}_REMOTE_$$$ext"
+	BASE="$MERGETOOL_TMPDIR/${BASE}_BASE_$$$ext"
 
 	base_mode=$(git ls-files -u -- "$MERGED" | awk '{if ($3==1) print $1;}')
 	local_mode=$(git ls-files -u -- "$MERGED" | awk '{if ($3==2) print $1;}')