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.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])