git-p4: add initial support for RCS keywords

git-p4: add initial support for RCS keywords

RCS keywords cause problems for git-p4 as perforce always
expands them (if +k is set) and so when applying the patch,
git reports that the files have been modified by both sides,
when in fact they haven't.

This change means that when git-p4 detects a problem applying
a patch, it will check to see if keyword expansion could be
the culprit. If it is, it strips the keywords in the p4
repository so that they match what git is expecting. It then
has another go at applying the patch.

This behaviour is enabled with a new git-p4 configuration
option and is off by default.

Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>