util/gitconfig: Refuse to commit on lint failure

After running `lint-stable` in the pre-commit hook, its result is
ignored. This behavior was introduced in commit b18f522b
(lint/gitconfig: Enable checkpatch.pl checking of commits) and it
doesn't seem intentional. This issue was also mentioned in the revert
discussion (https://review.coreboot.org/c/coreboot/+/17440).

Enable `errexit` mode of the shell so that the hook fails when an error
occurs in any of the tests. Also, enable `nounset` mode to catch typos
easier.

Change-Id: I749963167660ea6a1a04d40a14ad1113e82f0f86
Signed-off-by: Alex Thiessen <alex.thiessen.de+coreboot@gmail.com>
Reviewed-on: https://review.coreboot.org/23130
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

util/gitconfig/pre-commit

@@ -15,6 +15,9 @@
 ## GNU General Public License for more details.
 ##
 
+set -e # -o errexit
+set -u # -o nounset
+
 %MAKE% lint-stable
 
 PATCHDIFF=$(git diff --cached)