D1211: context: switch ctx() use to changectx()

[phillco (Phil Cohen)]

phillco added a subscriber: martinvonz.
phillco added a comment.


  I made https://phab.mercurial-scm.org/D1527 the new bottom of the stack, per @martinvonz's request. It actually caught a mistake that made me significantly change this commit :)

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D1211

To: phillco, #hg-reviewers
Cc: martinvonz, mercurial-devel