git-p4: test and document --use-client-spec

git-p4: test and document --use-client-spec

The depot path is required, even with this option.  Make sure
git-p4 fails and exits with non-zero.

Contents in the specified depot path will be rearranged according
to the client spec.  Test this and add a note in the docs.

Leave an XXX suggesting that this is somewhat confusing behavior
that might be good to fix later.

Function stripRepoPath() looks at self.useClientSpec.  Make sure
this is set both for command-line option --use-client-spec and
for configuration variable git-p4.useClientSpec.  Test this.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>