mergetool: Don't keep temporary merge files unless told to

This changes git mergetool to remove the temporary files used to invoke
the merge tool even if it returns non-zero.

This also adds a configuration option (mergetool.keepTemporaries) to
retain the previous behaviour if desired.

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

Documentation/config.txt

@@ -976,6 +976,13 @@ mergetool.keepBackup::
 	is set to `false` then this file is not preserved.  Defaults to
 	`true` (i.e. keep the backup files).
 
+mergetool.keepTemporaries::
+	When invoking a custom merge tool, git uses a set of temporary
+	files to pass to the tool. If the tool returns an error and this
+	variable is set to `true`, then these temporary files will be
+	preserved, otherwise they will be removed after the tool has
+	exited. Defaults to `false`.
+
 mergetool.prompt::
 	Prompt before each invocation of the merge resolution program.
 

git-mergetool.sh

@@ -269,6 +269,11 @@ merge_file () {
     if test "$status" -ne 0; then
 	echo "merge of $MERGED failed" 1>&2
 	mv -- "$BACKUP" "$MERGED"
+
+	if test "$merge_keep_temporaries" = "false"; then
+	    cleanup_temp_files
+	fi
+
 	return 1
     fi
 
@@ -415,6 +420,7 @@ else
     init_merge_tool_path "$merge_tool"
 
     merge_keep_backup="$(git config --bool merge.keepBackup || echo true)"
+    merge_keep_temporaries="$(git config --bool mergetool.keepTemporaries || echo false)"
 
     if test -z "$merge_tool_cmd" && ! type "$merge_tool_path" > /dev/null 2>&1; then
         echo "The merge tool $merge_tool is not available as '$merge_tool_path'"