Add Overcommit configuration

Enable the Gerrit change Id hook by default.

Change-Id: Ibe46eef9b6e26e9498fddd33f4b53f34fad164a7
Reviewed-on: http://gerrit.causes.com/36999
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>