D821: unamend: move fb extension unamend to core

[durin42 (Augie Fackler)]

durin42 added inline comments.

INLINE COMMENTS

> durham wrote in uncommit.py:265
> I'd just let unamend undo an unamend.  Letting unamend toggle back and forth between the two states seems like it might grant the user more confidence in the command, even.

Works for me. Add some documentation/testing of this so we know it's intentional in the future?

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D821

To: pulkit, #hg-reviewers, durham
Cc: quark, durin42, ryanmce, singhsrb, durham, mercurial-devel