Add test for installing overcommit

After fixing an issue with installing overcommit hooks in a repo with
hooks installed via template directory in 6d6f8536, we add some tests to
ensure we don't break this in future.

Change-Id: Ic3f31ecc288d1273ef17d3c9e7e12d97e72e0781
Reviewed-on: http://gerrit.causes.com/45271
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>