checkout: do not delete directories with untracked entries

If the `GIT_CHECKOUT_FORCE` flag is given to any of the `git_checkout`
invocations, we remove files which were previously staged. But while
doing so, we unfortunately also remove unstaged files in a directory
which contains at least one staged file, resulting in potential data
loss.

This commit adds two tests to verify behavior.

tests/checkout/head.c

@@ -60,3 +60,79 @@ void test_checkout_head__with_index_only_tree(void)
 
 	git_index_free(index);
 }
+
+void test_checkout_head__do_not_remove_untracked_file(void)
+{
+	git_checkout_options opts = GIT_CHECKOUT_OPTIONS_INIT;
+	git_index *index;
+
+	cl_git_pass(p_mkdir("testrepo/tracked", 0755));
+	cl_git_mkfile("testrepo/tracked/tracked", "tracked\n");
+	cl_git_mkfile("testrepo/tracked/untracked", "untracked\n");
+
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_add_bypath(index, "tracked/tracked"));
+	cl_git_pass(git_index_write(index));
+
+	git_index_free(index);
+
+	opts.checkout_strategy = GIT_CHECKOUT_FORCE;
+	cl_git_pass(git_checkout_head(g_repo, &opts));
+
+	cl_assert(!git_path_isfile("testrepo/tracked/tracked"));
+	cl_assert(git_path_isfile("testrepo/tracked/untracked"));
+}
+
+void test_checkout_head__do_not_remove_untracked_file_in_subdir(void)
+{
+	git_checkout_options opts = GIT_CHECKOUT_OPTIONS_INIT;
+	git_index *index;
+
+	cl_git_pass(p_mkdir("testrepo/tracked", 0755));
+	cl_git_pass(p_mkdir("testrepo/tracked/subdir", 0755));
+	cl_git_mkfile("testrepo/tracked/tracked", "tracked\n");
+	cl_git_mkfile("testrepo/tracked/subdir/tracked", "tracked\n");
+	cl_git_mkfile("testrepo/tracked/subdir/untracked", "untracked\n");
+
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_add_bypath(index, "tracked/tracked"));
+	cl_git_pass(git_index_add_bypath(index, "tracked/subdir/tracked"));
+	cl_git_pass(git_index_write(index));
+
+	git_index_free(index);
+
+	opts.checkout_strategy = GIT_CHECKOUT_FORCE;
+	cl_git_pass(git_checkout_head(g_repo, &opts));
+
+	cl_assert(!git_path_isfile("testrepo/tracked/tracked"));
+	cl_assert(!git_path_isfile("testrepo/tracked/subdir/tracked"));
+	cl_assert(git_path_isfile("testrepo/tracked/subdir/untracked"));
+}
+
+void test_checkout_head__do_remove_tracked_subdir(void)
+{
+	git_checkout_options opts = GIT_CHECKOUT_OPTIONS_INIT;
+	git_index *index;
+
+	cl_git_pass(p_mkdir("testrepo/subdir", 0755));
+	cl_git_pass(p_mkdir("testrepo/subdir/tracked", 0755));
+	cl_git_mkfile("testrepo/subdir/tracked-file", "tracked\n");
+	cl_git_mkfile("testrepo/subdir/untracked-file", "untracked\n");
+	cl_git_mkfile("testrepo/subdir/tracked/tracked1", "tracked\n");
+	cl_git_mkfile("testrepo/subdir/tracked/tracked2", "tracked\n");
+
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_add_bypath(index, "subdir/tracked-file"));
+	cl_git_pass(git_index_add_bypath(index, "subdir/tracked/tracked1"));
+	cl_git_pass(git_index_add_bypath(index, "subdir/tracked/tracked2"));
+	cl_git_pass(git_index_write(index));
+
+	git_index_free(index);
+
+	opts.checkout_strategy = GIT_CHECKOUT_FORCE;
+	cl_git_pass(git_checkout_head(g_repo, &opts));
+
+	cl_assert(!git_path_isdir("testrepo/subdir/tracked"));
+	cl_assert(!git_path_isfile("testrepo/subdir/tracked-file"));
+	cl_assert(git_path_isfile("testrepo/subdir/untracked-file"));
+}