D821: unamend: move fb extension unamend to core

[ryanmce (Ryan McElroy)]

ryanmce added inline comments.

INLINE COMMENTS

> durham wrote in uncommit.py:260
> This should be in a with statement probably?  Can we just have it be as part of the top level lock with statement?

Test test

REPOSITORY
  rHG Mercurial

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

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