repo: remove now-unused changectx() method (API)
repo.changectx(x) was just a synonym for repo[x], so any extensions
that fail due to this commit should switch over to that form.
Differential Revision: https://phab.mercurial-scm.org/D3037
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state