compat: adapt to change in mercurial core
The code around merge destination have been changed in core, we do some basic
adaptation to make sure we don't crash.
Further rework to adapt//take advantage of core change would probably be useful
but this is not the point of this commit.
# topics
Topics are an experiment to see if maybe the workflow defined by git
branches and hg bookmarks is only partially what users want - perhaps
something that feels more like a traditional VCS branch is right, but
that it should "dissolve" upon being finished. This extension exists
to be a sandbox for that experimentation.
# usage
Enable topics like any mercurial extension: download the source code to a
local directory, and add that directory to your `.hgrc`:
[extensions]
topics=path/to/hg-topics/src