Resolve "Merge button does not appear after Rebase-ing"

What does this MR do?

This Merge Request sets the cached SHA number on the merge request page after rebasing. This fixes a bug where the user previously would click the rebase button and see the shaMismatch state after the rebase finished. They will now see the merge button.

What are the relevant issue numbers?

Closes #47052 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines(https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html)
• Conforms to the style guides
• Security reports checked/validated by reviewer