Add overcommit.yml for the ChangeId hook

ScssLint is disabled by default because the current file doesn't pass
the linters.

(I wonder if there is a way we can make Gerrit suggest that people
install overcommit, instead of its default instructions to install a git
hook manually, directly from gerrit.)

Change-Id: I2a36a49f76f817f30756ec14d82229e19c79ebf1
Reviewed-on: http://gerrit.causes.com/40949
Reviewed-by: Joe Lencioni <joe.lencioni@brigade.com>
Tested-by: Tom Dooner <tom.dooner@brigade.com>