Add tests for git-prune

Add tests for git-prune

It seems that git prune changed behaviour with respect to revisions added
from command line, probably when it became a builtin. Currently, it prints
a short usage and exits: instead, it should take those revisions into
account and not prune them. So add a couple of test to point this out.

We'll be fixing this by switching to parse_options(), so add tests to
detect bogus command line parameters as well, to keep ourselves from
introducing regressions.

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>