`get_git_file_content` shallow-fetches the old binary's commit when it isn't in Evergreen's clone. That fetch had a single attempt, so a transient GitHub 503 failed the whole task (BF-45380).

Retries the fetch with tenacity: 5 attempts, exponential backoff capped at 30s. Only the network-dependent path retries; a genuinely bad commit hash still fails with git's stderr.

1. 1. 1. Testing
         Three unit tests in `test_generate_multiversion_exclude_tags.py` covering the no-fetch, transient-503-then-success, and persistent-failure paths. Full file passes (13 tests).