git apply --directory broken for new files

git apply --directory broken for new files

We carefully verify that the input to git-apply is sane,
including cross-checking that the filenames we see in "+++"
headers match what was provided on the command line of "diff
--git". When --directory is used, however, we ended up
comparing the unadorned name to one with the prepended root,
causing us to complain about a mismatch.

We simply need to prepend the root directory, if any, when
pulling the name out of the git header.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>