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 (closed).

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together