Cache Repository#ancestor? on two levels

What does this MR do?

We sometimes call this multiple times within a request for the MergeReqeustPollWidgetEntity.

• expose :should_be_rebased
  • MergeRequest#should_be_rebased?
    • MergeRequest#ff_merge_possible?
• expose :mergeable
  • MergeRequest#should_be_rebased?
• expose :rebase_path
  • MergeRequestPresenter#rebase_path
    • MergeRequest#should_be_rebased?

Sometimes the Gitaly call for this can take up to 10 seconds: https://log.gitlab.net/goto/01f164873d08893fb83529457a049622

Since the ancestor for a commit never changes, and the data to be stored is very small. It seems easy enough to cache this.

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8445#note_248074620 and gitlab-com/gl-infra/scalability#64 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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