Add a DVCS capability gated behind the DVCS feature

Review Request #9905 — Created May 2, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
6503100...

Reviewers

We now have the review_request.supports_history capability, which is
only present when the dvcs feature is enabled. When disabled, this
capability does not appear in the set of capabilities at all.

Ran unit tests.