Merge branch 'fix/limit-rev-list-count-to-one' into 'master'

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

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 v 8.11.0 (unreleased)
   - Fix of 'Commits being passed to custom hooks are already reachable when using the UI'
+  - Limit git rev-list output count to one in forced push check
 
 v 8.10.0 (unreleased)
   - Fix profile activity heatmap to show correct day name (eanplatter)

lib/gitlab/checks/force_push.rb

@@ -8,8 +8,8 @@ module Gitlab
         if Gitlab::Git.blank_ref?(oldrev) || Gitlab::Git.blank_ref?(newrev)
           false
         else
-          missed_refs, _ = Gitlab::Popen.popen(%W(#{Gitlab.config.git.bin_path} --git-dir=#{project.repository.path_to_repo} rev-list #{oldrev} ^#{newrev}))
-          missed_refs.split("\n").size > 0
+          missed_ref, _ = Gitlab::Popen.popen(%W(#{Gitlab.config.git.bin_path} --git-dir=#{project.repository.path_to_repo} rev-list --max-count=1 #{oldrev} ^#{newrev}))
+          missed_ref.present?
         end
       end
     end