Fire update hook from GitLab

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/10049
• Dev link: None
• GitLab version: 8.0

Expected behavior

Custom update hooks should be fired by GitLab when a commit is made via the UI.

Observed behavior

Only pre-receive and post-receive hooks are executed.

Next steps

Add support for update hook execution. @dzaporozhets and I discussed this already.

/cc @JobV