checkout test: ensure .gitattributes lifecycle

The .gitattributes cache should not reload .gitattributes in the
middle of checking out, only between checkout operations.  Otherwise,
we'll spend all our time stat'ing and read'ing the gitattributes.

tests/checkout/tree.c

@@ -1130,3 +1130,53 @@ void test_checkout_tree__can_collect_perfdata(void)
 
 	git_object_free(obj);
 }
+
+void update_attr_callback(
+	const char *path,
+	size_t completed_steps,
+	size_t total_steps,
+	void *payload)
+{
+	if (path && strcmp(path, "ident1.txt") == 0)
+		cl_git_write2file("testrepo/.gitattributes",
+			"*.txt ident\n", 12, O_RDWR|O_CREAT, 0666);
+}
+
+void test_checkout_tree__caches_attributes_during_checkout(void)
+{
+	git_checkout_options opts = GIT_CHECKOUT_OPTIONS_INIT;
+	git_oid oid;
+	git_object *obj = NULL;
+	git_buf ident1 = GIT_BUF_INIT, ident2 = GIT_BUF_INIT;
+	char *ident_paths[] = { "ident1.txt", "ident2.txt" };
+
+	opts.progress_cb = update_attr_callback;
+
+	assert_on_branch(g_repo, "master");
+	opts.checkout_strategy = GIT_CHECKOUT_FORCE;
+	opts.paths.strings = ident_paths;
+	opts.paths.count = 2;
+
+	cl_git_pass(git_reference_name_to_id(&oid, g_repo, "refs/heads/ident"));
+	cl_git_pass(git_object_lookup(&obj, g_repo, &oid, GIT_OBJ_ANY));
+
+	cl_git_pass(git_checkout_tree(g_repo, obj, &opts));
+
+	cl_git_pass(git_futils_readbuffer(&ident1, "testrepo/ident1.txt"));
+	cl_git_pass(git_futils_readbuffer(&ident2, "testrepo/ident2.txt"));
+
+	cl_assert_equal_strn(ident1.ptr, "# $Id$", 6);
+	cl_assert_equal_strn(ident2.ptr, "# $Id$", 6);
+
+	cl_git_pass(git_checkout_tree(g_repo, obj, &opts));
+
+	cl_git_pass(git_futils_readbuffer(&ident1, "testrepo/ident1.txt"));
+	cl_git_pass(git_futils_readbuffer(&ident2, "testrepo/ident2.txt"));
+
+	cl_assert_equal_strn(ident1.ptr, "# $Id: ", 7);
+	cl_assert_equal_strn(ident2.ptr, "# $Id: ", 7);
+
+	git_buf_free(&ident1);
+	git_buf_free(&ident2);
+	git_object_free(obj);
+}