This starts creating generic objects ("CommitRef", "BranchRef") which have attachable properties, like many Phabricator objects do. Here, they're formalized (and theoretically extensible), as "hardpoints".
So: a hardpoint is something on an object which you can attach stuff to, but which we don't start with the data for.
All of the logic for actually figuruing out how to attach stuff to hardpoints is also modular. Loader classes have code for loading stuff onto objects. For example, ArcanistMercurialBranchCommitHardpointLoader knows how to run hg log to build the commit for a branch.
One issue is that arc feature in Mercurial is 100% bookmarks, so maybe I should actually be making ArcanistRefRef here. But we can probbbably deal with that later.
Overall, I expect this will see some revision in future changes, and perhaps most of it will go away.