Merge branch '21010-emailsonpushworker-incorrectly-claims-deleted-a-commit-after-a-push' into 'master'

Resolve "EmailsOnPushWorker incorrectly claims deleted a commit after a push"

## What does this MR do?

Fix the comparison order in the emails on push worker, so regular pushes don't show a message about force pushing.

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

Don't think so, it was just a typo: `after_sha` should map to the `source_branch` argument, because it's the head of the comparison.

## Why was this MR needed?

To fix a regression!

## What are the relevant issue numbers?

Closes #21010.

## Does this MR meet the acceptance criteria?

- [x] ~~[CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
- [x] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [x] ~~API support added~~
- Tests
  - [x] 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)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5894

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