Summary:
We currently use the language "relative commit" or "relative local commit" to refer to the head of a commit range. I want to move toward callign this a "base commit", since I think that makes more sense. This moves us a small step in that direction.
The DSL stuff in T1233 also needs access to this stuff, so move it up to the base. This is mostly a bite-sized piece of the change in that diff.
Test Plan: Ran "arc which" in a couple of circumstances, read explanations.
Reviewers: dschleimer, vrana
Reviewed By: dschleimer
CC: aran
Maniphest Tasks: T1233
Differential Revision: https://secure.phabricator.com/D2747