Reduced the execution time of scripts/review_pull_requests.py by ~75%
(from ~1.5s to ~0.38s) for repositories with many branches.
- Implemented get_git_version() to check for ahead-behind atom support.
- Added BRANCH_METADATA_CACHE to store pre-fetched branch information.
- Replaced iterative 'git log' and 'git branch' calls with a single
'git for-each-ref' command using the %(ahead-behind:main) atom.
- Provided a robust fallback mechanism for Git versions < 2.41.
