clean: demonstrate a bug with pathspecs

b9670c1f5e (dir: fix checks on common prefix directory, 2019-12-19)
modified the way pathspecs are handled when handling a directory
during "git clean -f <path>". While this improved the behavior for
known test breakages, it also regressed in how the clean command
handles cleaning a specified file.

Add a test case that demonstrates this behavior. This test passes
before b9670c1f5e then fails after.

Helped-by: Kevin Willford <Kevin.Willford@microsoft.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7300-clean.sh

@@ -737,4 +737,13 @@ test_expect_success MINGW 'handle clean & core.longpaths = false nicely' '
 	test_i18ngrep "too long" .git/err
 '
 
+test_expect_failure 'clean untracked paths by pathspec' '
+	git init untracked &&
+	mkdir untracked/dir &&
+	echo >untracked/dir/file.txt &&
+	git -C untracked clean -f dir/file.txt &&
+	ls untracked/dir >actual &&
+	test_must_be_empty actual
+'
+
 test_done