add gerrit_commit_msg_hook

this is a verbatim copy of what we get from gerrit.
the idea is to make it directly available as part of a checkout, so we
can better automate the setup via init-repository, and we can keep
people updated automatically.

Change-Id: I2e46666cb6d18cac9fc060870d020d68935e2282
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>