Summary:
- Replace SVN-specific language with VCS-agnostic language.
- Add new "arc close-revision", works exactly like "arc mark-committed" but with agnostic language.
- Use status constants, not status strings.
- Mark "arc mark-committed" deprecated.
- Remove deprecated "arc merge".
Test Plan: Ran "arc mark-committed", "arc close-revision".
Reviewers: btrahan, vrana, jungejason
Reviewed By: btrahan
CC: aran, Makinde
Maniphest Tasks: T909
Differential Revision: https://secure.phabricator.com/D2244
