D821: unamend: move fb extension unamend to core

[durham (Durham Goode)]

durham requested changes to this revision.
durham added a comment.
This revision now requires changes to proceed.


  Generally looks good.  Just need to fix the transaction thing.

REPOSITORY
  rHG Mercurial

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

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