Rebuild "arc branch" on new "hardpoint" infrastructure

Summary:
Ref T11355. Ref T10895. Ref T11518. This is heading to experiemntal. This may or may not be a good idea, but basically it's a more generic version of Query classes in Phabricator.

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.

This moves us somewhat closer to T11355 and T11518, although the immediate thing I want to do with it is define an ArcanistObjectNameRef and use hardpoints to load URIs for it for T10895.

Overall, I expect this will see some revision in future changes, and perhaps most of it will go away.

Test Plan: Ran arc branch / arc feature in Git and Mercurial repositories.

Reviewers: avivey, chad

Reviewed By: chad

Maniphest Tasks: T10895, T11518, T11355

Differential Revision: https://secure.phabricator.com/D16857