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.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html