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>

util/gitconfig/pre-commit

@@ -20,7 +20,7 @@ set -u # -o nounset
 
 %MAKE% lint-stable
 
-PATCHDIFF=$(git diff --cached)
+PATCHDIFF=$(git diff --cached --src-prefix=a/ --dst-prefix=b/)
 if printf "%s\n" "$PATCHDIFF" | grep -q "@@"; then
 	echo
 	echo "Running checkpatch"