Branch is no longer checked out when using refspec

Summary

Original discussion started in https://forum.gitlab.com/t/src-refspec-master-does-not-match-any/24691 all the jobs that were posted was because they are trying to push a specific branch. Using refspec we don't check out a branch but checkout a specific commit.

Steps to reproduce

Have a job trying to push master

deploy:
  stage: deploy
  script:
  - git remote add heroku https://heroku:$HEROKU_API_KEY@git.heroku.com/<project>.git && git push -f heroku master
  environment:
    name: production
    url: https://<project>.herokuapp.com
  only:
  - master

Actual behavior

A user would get the error Src refspec master does not match any

Expected behavior

The user can push master to any remote.

Relevant logs and/or screenshots

A job running git branch as part of the script.

• Runner 11.9
• Runner 11.8

Used GitLab Runner version

11.9

Regression most likely introduced in !1203 (merged)

Workaround

Use HEAD when you want to push this to another remote.

deploy:
  stage: deploy
  script:
  - git remote add heroku https://heroku:$HEROKU_API_KEY@git.heroku.com/<project>.git
  - git push -f heroku HEAD:master

Wont Fix ⚠

At first, we were implementing !1270 (merged) as a fix but decided to revert it in !1283 (merged) since it had some bad performance regressions. The behavior we had was flaky at best for the following reasons described in #4097 (comment 156508723)

/cc @nolith @tmaczukin @erushton