clean: unreadable directory may still be rmdir-able if it is empty

As a last ditch effort, try rmdir(2) when we cannot read the directory
to be removed.  It may be an empty directory that we can remove without
any permission, as long as we can modify its parent directory.

Noticed by Linus.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7300-clean.sh

@@ -453,4 +453,11 @@ test_expect_success 'git clean -e' '
 	)
 '
 
+test_expect_success SANITY 'git clean -d with an unreadable empty directory' '
+	mkdir foo &&
+	chmod a= foo &&
+	git clean -dfx foo &&
+	! test -d foo
+'
+
 test_done