rollback_lock_file(): exit early if lock is not active

Eliminate a layer of nesting.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

lockfile.c

@@ -276,10 +276,11 @@ int hold_locked_index(struct lock_file *lk, int die_on_error)
 
 void rollback_lock_file(struct lock_file *lk)
 {
-	if (lk->filename[0]) {
-		if (lk->fd >= 0)
-			close(lk->fd);
-		unlink_or_warn(lk->filename);
-		lk->filename[0] = 0;
-	}
+	if (!lk->filename[0])
+		return;
+
+	if (lk->fd >= 0)
+		close(lk->fd);
+	unlink_or_warn(lk->filename);
+	lk->filename[0] = 0;
 }