Add basic coverage to GitHook class

For ease of refactoring, and because it's The Right Thing To Do.

Change-Id: I73be62fb97fbd15a6c3d6cc5a402425c35ce444b
Reviewed-on: https://gerrit.causes.com/22957
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>