tests/test-backwards-remove.t

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.

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls
  a