util/gitconfig: improve robustness of checkpatch in pre-commit

util/gitconfig: improve robustness of checkpatch in pre-commit

Users can have non-default configurations as to how git diff et al are
presenting file names in diffs (default: a/ and b/ prefixes). checkpatch
expects that and trims the first element, so enforce that configuration
for the diff that's sent into it.

Change-Id: I099795119456a73c900b31ce191c2d9e898a5c7e
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/26694
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Martin Roth <martinroth@google.com>