Fix diverged from upstream detection with pull_mirror_branch_prefix

What does this MR do?

The current Repository#diverged_from_upstream? and Repository#up_to_date_with_upstream? implementations don't catch diverged branches when pull_mirror_branch_prefix is set. (see !17368 (merged))

This MR changes how those methods searches commits upstream.

It also adds more tests to the pull_mirror_branch_prefix feature

related to gitlab-com/gl-infra/delivery#472 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - not needed, fixing a feature under feature flag
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers