git-clean buglet

Try this in your favorite git repo:

    git clean -n /

Here, it responds with:

    fatal: oops in prep_exclude

I don't know what the expected behavior should be, but certainly not this.
Maybe just do nothing like 'git ls-files /'.

I'm not familiar with either builtin-clean.c nor dir.c; perhaps someone
else can fix this?

-- Hannes