Fix 'Object not found - no match for id (sha)' when importing GitHub PRs

What does this MR do?

When the branch exists, check if the sha is part of the branch we found, otherwise fallback to create a new branch with a pattern we already use to import removed branches.

Are there points in the code the reviewer needs to double check?

Gitlab::GithubImporter::BranchFormatter#commit_exists?

Why was this MR needed?

There are some edges cases where PRs were not imported.

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #29093 (closed)

cc @brodock @xiaogang_gitlab @balameb