Don't explicitly name abstract base classes

Summary:
Ref T5655. It is superfluous to include "base" in the name of an abstract base class. Furthermore, it is not done consistently within the code base.

In order to retain compatibility with external code, I have kept the `ArcanistBaseWorkflow` class (which trivially extends from `ArcanistWorkflow`), but it is now deprecated and should output a warning message. Similarly for `ArcanistBaseUnitTestEngine`.

Test Plan: Created a workflow which extends from `ArcanistBaseWorkflow`. Executed the workflow and saw a deprecation warning.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin, aurelijus

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D9983
Restore the old "history.immutable" default behavior

Summary: See rARC104219dd. Per the explanatory text, this should default to `true` in Mercurial and `false` in Git.

Test Plan: Ran `arc get-config` in Mercurial repo.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10021
Merge branch 'master' of github.com:facebook/arcanist