Display last commit of deleted branch in push events

What does this MR do?

Display the last commit of a deleted branch in the push events of a project.

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

Is the change in app/models/event.rb the correct way to display a two-line event for deleted branches?

Why was this MR needed?

It is easier to restore an accidentally deleted branch if the commit hash is displayed in the push events.

What are the relevant issue numbers?

Fixes #18659 (closed)

Screenshots

Before garbage collection

After garbage collection