feat(check-merge-safety): fallback to git command when diff fails

[bensbigolbeard]

📝 Description

• adds fallback for diff attempts through the compare endpoint, which fails with "diff too large" errors for large repos with many deltas between SHAs.
• the change is to first attempt the existing diff call, then when you receive a 406 (diff too large error code), fetch each sha and use the local git installation to run the diff.
• new failure states:
  • when diff fails with 40x errors, via octokit.repos.compareCommitsWithBasehead, the catch block processes matching cases, else it's thrown again to match current handling
  • when individual branches/SHAs cannot be retrieved via git.fetch
  • when diffs between two SHAs fail via git.diff

🔗 Related Issues

[bensbigolbeard]

closing in favor of #702