rollback_lock_file(): set fd to -1

When rolling back the lockfile, call close_lock_file() so that the
lock_file's fd field gets set back to -1. This keeps the lock_file
object in a valid state, which is important because these objects are
allowed to be reused. It also makes it unnecessary to check whether
the file has already been closed, because close_lock_file() takes care
of that.

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

lockfile.c

@@ -279,8 +279,7 @@ void rollback_lock_file(struct lock_file *lk)
 	if (!lk->filename[0])
 		return;
 
-	if (lk->fd >= 0)
+	close_lock_file(lk);
-		close(lk->fd);
 	unlink_or_warn(lk->filename);
 	lk->filename[0] = 0;
 }