gitconfig: Add lint-stable as pre-commit hook

When configuring the tree with "make gitconfig", a pre-commit hook
is installed that runs the stable lint tests.
If any of these fail, the log is visible (on stdout) and the
commit is aborted.

Change-Id: Ie2a26e87f466c63b24db8dca8827057a18ac7f3e
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/682
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>