Correct force_write bug in refs.c

My earlier attempt at forcing a write for non-existant refs worked;
it forced a write for pretty much all refs.  This corrects the
condition to only force a write for refs which don't exist yet.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

refs.c

@@ -305,7 +305,7 @@ static struct ref_lock* lock_ref_sha1_basic(const char *path,
 	lock->ref_file = strdup(path);
 	lock->lock_file = strdup(mkpath("%s.lock", lock->ref_file));
 	lock->log_file = strdup(git_path("logs/%s", lock->ref_file + plen));
-	lock->force_write = !lstat(lock->ref_file, &st) || errno == ENOENT;
+	lock->force_write = lstat(lock->ref_file, &st) && errno == ENOENT;
 
 	if (safe_create_leading_directories(lock->lock_file))
 		die("unable to create directory for %s", lock->lock_file);