[RFC] Second attempt at making git-clean a builtin

I've taken all of the comments I received from my previous attempt see:

http://marc.info/?l=git&m=119181975419521&w=2

With these new changes in place my new git-clean passes all of the
original tests as well as the new tests I've added.  While looking at
how git-ls-files walks the tree there were some things that didn't quite
understand, or thought might be unnecessary so there may be some things I
missed.  For example I'm still not quite sure what verify_pathspec()
does.

I did however notice what I would call a bug in the behavior of
git-ls-files and therefore the current git-clean.sh.  With the current
git-clean if you have two directories that contain only untracked files,
for example docs/ and examples/ running:

git clean docs/ examples/

will not remove either directory.  Instead you must use the -d
parameter.  To me this makes sense, however if you run:

git clean docs/

it will remove the docs directory without using the -d parameter.  My
patch is at least consistent in that it requires the -d in both cases.