remote: create FETCH_HEAD with a refspecless remote

When downloading the default branch due to lack of refspecs, we still
need to write out FETCH_HEAD with the tip we downloaded, unfortunately
with a format that doesn't match what we already have.

src/fetchhead.c

@@ -74,6 +74,7 @@ static int fetchhead_ref_write(
 {
 	char oid[GIT_OID_HEXSZ + 1];
 	const char *type, *name;
+	int head = 0;
 
 	assert(file && fetchhead_ref);
 
@@ -87,11 +88,16 @@ static int fetchhead_ref_write(
 		GIT_REFS_TAGS_DIR) == 0) {
 		type = "tag ";
 		name = fetchhead_ref->ref_name + strlen(GIT_REFS_TAGS_DIR);
+	} else if (!git__strcmp(fetchhead_ref->ref_name, GIT_HEAD_FILE)) {
+		head = 1;
 	} else {
 		type = "";
 		name = fetchhead_ref->ref_name;
 	}
 
+	if (head)
+		return git_filebuf_printf(file, "%s\t\t%s\n", oid, fetchhead_ref->remote_url);
+
 	return git_filebuf_printf(file, "%s\t%s\t%s'%s' of %s\n",
 		oid,
 		(fetchhead_ref->is_merge) ? "" : "not-for-merge",

src/remote.c

@@ -1032,6 +1032,27 @@ int git_remote_update_tips(git_remote *remote)
 			goto out;
 	}
 
+	/* If we have no refspecs, update HEAD -> FETCH_HEAD manually */
+	if (remote->refspecs.length == 0 && refs.length > 0 && git_remote_update_fetchhead(remote)) {
+		git_vector vec;
+		git_refspec headspec;
+
+		if (git_refspec__parse(&headspec, "HEAD", true) < 0)
+			goto out;
+
+		if (git_vector_init(&vec, 1, NULL) < 0) {
+			goto out;
+		}
+
+		if (git_vector_insert(&vec, git_vector_get(&refs, 0)) < 0) {
+			git_vector_free(&vec);
+			goto out;
+		}
+
+		error = git_remote_write_fetchhead(remote, &headspec, &vec);
+		git_vector_free(&vec);
+	}
+
 out:
 	git_refspec__free(&tagspec);
 	git_vector_free(&refs);