refs: failing test for concurrent ref access

If we remove a reference while we're iterating through the packed refs,
the position in the iterator will be off.

tests/refs/iterator.c

@@ -186,3 +186,36 @@ void test_refs_iterator__foreach_name_can_cancel(void)
 		-333);
 	cl_assert_equal_i(0, cancel_after);
 }
+
+void test_refs_iterator__concurrent_delete(void)
+{
+	git_reference_iterator *iter;
+	size_t full_count = 0, concurrent_count = 0;
+	const char *name;
+	int error;
+
+	git_repository_free(repo);
+	repo = cl_git_sandbox_init("testrepo");
+
+	cl_git_pass(git_reference_iterator_new(&iter, repo));
+	while ((error = git_reference_next_name(&name, iter)) == 0) {
+		full_count++;
+	}
+
+	git_reference_iterator_free(iter);
+	cl_assert_equal_i(GIT_ITEROVER, error);
+
+	cl_git_pass(git_reference_iterator_new(&iter, repo));
+	while ((error = git_reference_next_name(&name, iter)) == 0) {
+		cl_git_pass(git_reference_remove(repo, name));
+		concurrent_count++;
+	}
+
+	git_reference_iterator_free(iter);
+	cl_assert_equal_i(GIT_ITEROVER, error);
+
+	cl_assert_equal_i(full_count, concurrent_count);
+
+	cl_git_sandbox_cleanup();
+	repo = NULL;
+}