Disable progress bars for non-interactive sessions

[Mirochill]

Summary

• route the shape comparison progress bars through a small helper
• disable those tqdm bars automatically when stderr is not an interactive terminal
• add targeted coverage for interactive and non-interactive stderr handling

Fixes #126.

Validation

• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD
• Not run locally: test suite

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