Moved call of SystemHooksService from UpdateMergeRequestsWorker to GitPushServic…

What does this MR do?

I've moved the call of SystemHooksService from UpdateMergeRequestsWorker to GitPushService, seems more logically correct to have the push hook executed (both webhook and system hook) inside this service instead of the UpdateMergeRequestWorker. This allow to have the same output json in both Webhook and System hook push event. Actually, the System push hook json doesn't have the commit list inside. No rspec tests was done, so I know it's failing, tests can be added if this MR is interesting for reviewers.

I've checked some tasks below that I think are done, feel free to change them.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The json created from webhook and system hook for push event are different. It can be useful to have the list of commits also in system hook json.

Screenshots (if relevant)

Highlighted in red the difference of sent json between before and after this change.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?