Allow builtin-fetch to work on a detached HEAD

If we are running fetch in a repository that has a detached HEAD
then there is no current_branch available.  In such a case any ref
that the fetch might update by definition cannot also be the current
branch so we should always bypass the "don't update HEAD" test.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fetch.c

@@ -151,7 +151,8 @@ static int update_local_ref(struct ref *ref,
 		return 0;
 	}
 
-	if (!strcmp(ref->name, current_branch->name) &&
+	if (current_branch &&
+	    !strcmp(ref->name, current_branch->name) &&
 	    !(update_head_ok || is_bare_repository()) &&
 	    !is_null_sha1(ref->old_sha1)) {
 		/*