fix: handle corrupt git index errors in sanity check

[Kcstring]

Summary

• catch non-GitPython exceptions raised while probing tracked files in sanity_check_repo()
• surface corrupt git index errors as the existing repository corruption message instead of crashing
• add a regression test for the struct.error path seen with damaged indexes

Testing

• python3 -m pytest tests/basic/test_sanity_check_repo.py -k "corrupt_git_index or corrupt_repo or git_index_version_greater_than_2"
• python3 -m ruff check aider/main.py tests/basic/test_sanity_check_repo.py

Fixes #5061