fetch: test when HEAD points to nonexistent branch

When HEAD points to a nonexistent or invalid branch - for example, to
`refs/heads/` - the fetch should be permitted to continue, but we should
not use it when creating the `for merge` option in the FETCH_HEAD file.

(This emulates git's behavior.)

tests/fetchhead/nonetwork.c

@@ -319,6 +319,16 @@ static int assert_master_for_merge(const char *ref, const char *url, const git_o
 	return 0;
 }
 
+static int assert_none_for_merge(const char *ref, const char *url, const git_oid *id, unsigned int is_merge, void *data)
+{
+	GIT_UNUSED(ref);
+	GIT_UNUSED(url);
+	GIT_UNUSED(id);
+	GIT_UNUSED(data);
+
+	return is_merge ? -1 : 0;
+}
+
 void test_fetchhead_nonetwork__unborn_with_upstream(void)
 {
 	git_repository *repo;
@@ -366,6 +376,25 @@ void test_fetchhead_nonetwork__fetch_into_repo_with_symrefs(void)
 	cl_git_sandbox_cleanup();
 }
 
+void test_fetchhead_nonetwork__fetch_into_repo_with_invalid_head(void)
+{
+	git_remote *remote;
+	char *strings[] = { "refs/heads/*:refs/remotes/origin/*" };
+	git_strarray refspecs = { strings, 1 };
+
+	cl_set_cleanup(&cleanup_repository, "./test1");
+	cl_git_pass(git_repository_init(&g_repo, "./test1", 0));
+
+	/* HEAD pointing to nonexistent branch */
+	cl_git_rewritefile("./test1/.git/HEAD", "ref: refs/heads/\n");
+
+	cl_git_pass(git_remote_create_anonymous(&remote, g_repo, cl_fixture("testrepo.git")));
+	cl_git_pass(git_remote_fetch(remote, &refspecs, NULL, NULL));
+	cl_git_pass(git_repository_fetchhead_foreach(g_repo, assert_none_for_merge, NULL));
+
+	git_remote_free(remote);
+}
+
 void test_fetchhead_nonetwork__quote_in_branch_name(void)
 {
 	cl_set_cleanup(&cleanup_repository, "./test1");