remote: append to FETCH_HEAD rather than overwrite for each refspec

We treat each refspec on its own, but the code currently overwrites the contents
of FETCH_HEAD so we end up with the entries for the last refspec we processed.

Instead, truncate it before performing the updates and append to it when
updating the references.

src/fetchhead.c

@@ -118,7 +118,7 @@ int git_fetchhead_write(git_repository *repo, git_vector *fetchhead_refs)
 	if (git_buf_joinpath(&path, repo->gitdir, GIT_FETCH_HEAD_FILE) < 0)
 		return -1;
 
-	if (git_filebuf_open(&file, path.ptr, GIT_FILEBUF_FORCE, GIT_REFS_FILE_MODE) < 0) {
+	if (git_filebuf_open(&file, path.ptr, GIT_FILEBUF_APPEND, GIT_REFS_FILE_MODE) < 0) {
 		git_buf_free(&path);
 		return -1;
 	}

src/remote.c

@@ -1541,6 +1541,20 @@ cleanup:
 	return error;
 }
 
+static int truncate_fetch_head(const char *gitdir)
+{
+	git_buf path = GIT_BUF_INIT;
+	int error;
+
+	if ((error = git_buf_joinpath(&path, gitdir, GIT_FETCH_HEAD_FILE)) < 0)
+		return error;
+
+	error = git_futils_truncate(path.ptr, GIT_REFS_FILE_MODE);
+	git_buf_free(&path);
+
+	return error;
+}
+
 int git_remote_update_tips(
 		git_remote *remote,
 		const git_remote_callbacks *callbacks,
@@ -1571,6 +1585,9 @@ int git_remote_update_tips(
 	else
 		tagopt = download_tags;
 
+	if ((error = truncate_fetch_head(git_repository_path(remote->repo))) < 0)
+		goto out;
+
 	if (tagopt == GIT_REMOTE_DOWNLOAD_TAGS_ALL) {
 		if ((error = update_tips_for_spec(remote, callbacks, update_fetchhead, tagopt, &tagspec, &refs, reflog_message)) < 0)
 			goto out;