Add post-update Git hook

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Summary

One of our customers has asked that we support the post-update Git hook in GitLab and GitLab-shell

Additional Detail

The customer requires the post-update hook to maintain their current Git workflow that cannot be accomplished by using webhooks

Relevant Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/29817

Customer has kindly provided an implementation of the post-update hook and changes necessary to GitlabCustomHook for GitLab-shell

Issue also raised against GitLab-shell for this functionality: gitlab-shell#49 (closed)

/cc @DouweM @jacobvosmaer-gitlab