Render "arc markers" workflows as a tree, not a list

Summary:
Ref T13546. Currently, each "land" workflow executes custom graph queries to find commits: move toward abstracting this logic.

The "land" workflow also has a potentially dangerous behavior: if you have "master > A > B > C" and "arc land C", it will land A, B, and C. However, an updated version of A or B may exist elsewhere in the working copy. If it does, "arc land" will incorrectly land an out-of-date set of changes.

To find newer versions of "A" and "B", we need to search backwards from all local markers to the nearest outgoing marker, then compare the sets of changes we find to the sets of changes selected by "arc land".

This is also roughly the workflow that "arc branches", etc., need to show local markers as a tree, and starting in "arc branches" allows the process to be visualized.

As implemented here ,this rendering is still somewhat rough, and the selection of "outgoing markers" isn't good. In Mercurial, we may plausibly be able to use phase markers, but in Git we likely can't guess the right behavior automatically and probably need additional configuration.

Test Plan: Ran "arc branches" and "arc bookmarks" in Git and Mercurial.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21363