cvsimport: fix relative argument filenames

cvsimport: fix relative argument filenames

One of the first things that cvsimport does is chdir to the
newly created git repo. This means that any filenames given
to us on the command line will be looked up relative to the
git repo directory. This is probably not what the user
expects, so let's remember and prepend the original
directory for relative filenames.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>