amend: wrap all commit operations in a single transaction
This allows proper recovery of an interrupted amend process.
No changes are made to the logic besides:
- indent operations into a single try-except clause,
- some comment and code wrapping to 80 chars,
- strip logic should not be contained in the transaction and is extracted from
the main code.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null