remote: don't resolve HEAD in non-repository

The remote-config code wants to look at HEAD to mark the
current branch specially. But if we are not in a repository
(e.g., running "git archive --remote"), this makes no sense;
there is no HEAD to look at, and we have no current branch.

This doesn't really cause any bugs in practice (if you are
not in a repo, you probably don't have a .git/HEAD file),
but we should be more careful about triggering the refs code
at all in a non-repo. As we grow new ref backends, we would
not even know which backend to use.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -455,18 +455,19 @@ static void read_config(void)
 {
 	static int loaded;
 	struct object_id oid;
-	const char *head_ref;
 	int flag;
 
 	if (loaded)
 		return;
 	loaded = 1;
 
 	current_branch = NULL;
-	head_ref = resolve_ref_unsafe("HEAD", 0, oid.hash, &flag);
-	if (head_ref && (flag & REF_ISSYMREF) &&
-	    skip_prefix(head_ref, "refs/heads/", &head_ref)) {
-		current_branch = make_branch(head_ref, 0);
+	if (startup_info->have_repository) {
+		const char *head_ref = resolve_ref_unsafe("HEAD", 0, oid.hash, &flag);
+		if (head_ref && (flag & REF_ISSYMREF) &&
+		    skip_prefix(head_ref, "refs/heads/", &head_ref)) {
+			current_branch = make_branch(head_ref, 0);
+		}
 	}
 	git_config(handle_config, NULL);
 	alias_all_urls();