clone: support remotes with references but no branches

A repository can have any number of references which we're not
interested in such as notes or tags. For the default branch calculation
we only care about branches. Make the decision about the number of
branches rather than the number of refs in general.

src/clone.c

@@ -138,6 +138,23 @@ static int update_head_to_new_branch(
 	return error;
 }
 
+/**
+ * Check whether there are any branches among the listed
+ * references. It's possible for a repository to have a long list of
+ * references without us downloading any of them.
+ */
+static bool remote_has_branches(const git_remote_head **refs, size_t len)
+{
+	size_t i;
+
+	for (i = 0; i < len; i++) {
+		if (!git__prefixcmp(refs[i]->name, GIT_REFS_HEADS_DIR))
+			return true;
+	}
+
+	return false;
+}
+
 static int update_head_to_remote(
 		git_repository *repo,
 		git_remote *remote,
@@ -156,7 +173,7 @@ static int update_head_to_remote(
 		return error;
 
 	/* Did we just clone an empty repository? */
-	if (refs_len == 0)
+	if (!remote_has_branches(refs, refs_len))
 		return setup_tracking_config(
 			repo, "master", GIT_REMOTE_ORIGIN, GIT_REFS_HEADS_MASTER_FILE);
 
@@ -167,7 +184,7 @@ static int update_head_to_remote(
 		found_branch = 1;
 	}
 
-	/* Get the remote's HEAD. This is always the first ref in the list. */
+	/* Get the remote's HEAD. This is always the first ref in the list if it exists */
 	remote_head = refs[0];
 	assert(remote_head);