WIP: Reduce amount of stored keep arounds

What does this MR do?

This tries to normalize keep around sha to HEAD sha and retain only HEAD sha when needed.

The tricky part is that #ancestor? is an expensive call to do, but maybe it is worth the cost.

This gonna work beautifully in the following conditions:

1. when we import project,
2. we import a lot of PRs,
3. since most of PRs are merged we do not need to create most of keep-around are part of HEAD.

What are the relevant issue numbers?

TBD

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer