Limit git rev-list output count to one in forced push check

## What does this MR do?
It adds `--max-count=1` to the git command used to check for forced push, this way we can have a brief yet indicative output, saving some time used to split the string returned.

Credits go to @pacoguzman for pointing me to the command option.

## Are there points in the code the reviewer needs to double check?
Don't think so.

## Why was this MR needed?
It could allow for faster check of forced pushes.

## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/14280#note_13177575

## Screenshots (if relevant)
N/A

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- ~~Tests~~
  - [ ] ~~Added for this feature/bug~~
  - [ ] ~~All builds are passing~~
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] ~~[Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)~~

See merge request !5352