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
## Screenshots
### Before garbage collection
![before-gc](/uploads/5674cd53e1564d48b7f2f8406ea0fbed/before-gc.png)
### After garbage collection
![after-gc](/uploads/80950c1932feeb3b69d0fc11b8f7acf4/after-gc.png)
See merge request !4699