Tighten git helper skip behavior

[codeforester]

Summary

• Correct git_update_repo usage text and make branch-mismatch skips return success.
• Demote default local-ref guidance in check_script_up_to_date to debug logging.
• Add git-helper regression coverage for usage text, skip behavior, invalid retry values, and debug-only local-ref guidance.
• Document why git_update_repo still changes into the target repo while running.

Validation

• bats lib/bash/git/tests/lib_git.bats
• ./tests/validate.sh
• git diff --check

Fixes #40