Call overcommit executable directly in integration spec

Since this is an integration test, we should execute the codepath via
CLI rather than via Ruby code.

This was discovered in the process of writing another test. Since we
cache `git_dir` and `repo_root` of the `Overcommit::Utils` module,
invoking this via Ruby could result in two separate invocations caching
these values from a previous call. Running via CLI prevents this as it
has its own Ruby process.

Change-Id: Ica3d8103c24a93ca1329cbae1a7a07162337cbe8
Reviewed-on: http://gerrit.causes.com/45270
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Tested-by: jenkins <jenkins@brigade.com>