git-stash: remove untracked/ignored directories when stashed

The two new stash options --include-untracked and --all do not remove the
untracked and/or ignored files that are stashed if those files reside in
a subdirectory. e.g. the following sequence fails:

mkdir untracked &&
echo hello >untracked/file.txt &&
git stash --include-untracked &&
test ! -f untracked/file.txt

Within the git-stash script, git-clean is used to remove the
untracked/ignored files, but since the -d option was not supplied, it does
not remove directories.

So, add -d to the git-clean arguments, and update the tests to test this
functionality.

Reported-by: Hilco Wijbenga <hilco.wijbenga@gmail.com>
Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>