checkout: Teach checkout to cope with orphaned Head

Fix #1236

src/checkout.c

@@ -1173,7 +1173,14 @@ static int checkout_data_init(
 
 	if (!data->opts.baseline) {
 		data->opts_free_baseline = true;
-		if ((error = checkout_lookup_head_tree(&data->opts.baseline, repo)) < 0)
+		error = checkout_lookup_head_tree(&data->opts.baseline, repo);
+
+		if (error == GIT_EORPHANEDHEAD) {
+			error = 0;
+			giterr_clear();
+		}
+
+		if (error < 0)
 			goto cleanup;
 	}
 

tests-clar/checkout/index.c

@@ -498,3 +498,13 @@ void test_checkout_index__can_update_prefixed_files(void)
 	cl_assert(!git_path_exists("testrepo/branch_file"));
 	cl_assert(!git_path_exists("testrepo/branch_file.txt.after"));
 }
+
+void test_checkout_index__can_checkout_a_newly_initialized_repository(void)
+{
+	test_checkout_index__cleanup();
+
+	g_repo = cl_git_sandbox_init("empty_standard_repo");
+	cl_git_remove_placeholders(git_repository_path(g_repo), "dummy-marker.txt");
+
+	cl_git_pass(git_checkout_index(g_repo, NULL, NULL));
+}