Plug a memleak in builtin-revert

Probably happened when working around git_path's problem with returned
buffer being reused.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-revert.c

@@ -269,7 +269,7 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 	int i;
 	char *oneline, *reencoded_message = NULL;
 	const char *message, *encoding;
-	const char *defmsg = xstrdup(git_path("MERGE_MSG"));
+	char *defmsg = xstrdup(git_path("MERGE_MSG"));
 
 	git_config(git_default_config, NULL);
 	me = action == REVERT ? "revert" : "cherry-pick";
@@ -426,6 +426,7 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 		return execv_git_cmd(args);
 	}
 	free(reencoded_message);
+	free(defmsg);
 
 	return 0;
 }