ignore: treat paths with trailing "/" as directories

The function `git_ignore_path_is_ignored` is there to test the
ignore status of paths that need not necessarily exist inside of
a repository. This has the implication that for a given path, we
cannot always decide whether it references a directory or a file,
and we need to distinguish those cases because ignore rules may
treat those differently. E.g. given the following gitignore file:

    *
    !/**/

we'd only want to unignore directories, while keeping files
ignored. But still, calling `git_ignore_path_is_ignored("dir/")`
will say that this directory is ignored because it treats "dir/"
as a file path.

As said, the `is_ignored` function cannot always decide whether
the given path is a file or directory, and thus it may produce
wrong results in some cases. While this is unfixable in the
general case, we can do better when we are being passed a path
name with a trailing path separator (e.g. "dir/") and always
treat them as directories.

src/ignore.c

@@ -534,7 +534,9 @@ int git_ignore_path_is_ignored(
 	memset(&path, 0, sizeof(path));
 	memset(&ignores, 0, sizeof(ignores));
 
-	if (git_repository_is_bare(repo))
+	if (!git__suffixcmp(pathname, "/"))
+		dir_flag = GIT_DIR_FLAG_TRUE;
+	else if (git_repository_is_bare(repo))
 		dir_flag = GIT_DIR_FLAG_FALSE;
 
 	if ((error = git_attr_path__init(&path, pathname, workdir, dir_flag)) < 0 ||

tests/attr/ignore.c

@@ -397,3 +397,19 @@ void test_attr_ignore__ignored_subdirfiles_with_negations(void)
 	assert_is_ignored(true, "dir/sub1/c.test");
 }
 
+void test_attr_ignore__negative_directory_rules_only_match_directories(void)
+{
+	cl_git_rewritefile(
+		"attr/.gitignore",
+		"*\n"
+		"!/**/\n"
+		"!*.keep\n"
+		"!.gitignore\n"
+	);
+
+	assert_is_ignored(true, "src");
+	assert_is_ignored(true, "src/A");
+	assert_is_ignored(false, "src/");
+	assert_is_ignored(false, "src/A.keep");
+	assert_is_ignored(false, ".gitignore");
+}