WIP - Custom hooks POC

The `custom_hooks` directory can be created within any repository directory. For example, in the existing `repos_path` under `namespace/project.git/` create a `custom_hooks` directory. Then place the proper git hook (currently pre-receive, post-receive or update) file in this directory. GitLab Shell will automatically invoke with the proper arg format for the specific hook.

## What's missing?
1. Other hooks types - pre-commit/etc
Redundant `return` detected.