Add Mercurial support for DVCS features

Review Request #10263 — Created Oct. 23, 2018 and submitted — Latest diff uploaded

Review Board

Mercurial repositories can now have review requests with commit
histories created. As part of this change, we now keep track of whether
or not a given SCM has a notion of committers vs authors (e.g., Git has
both committers and authors but Mercurial only has authors). In the case
where the SCM does not support committers, the relevant fields from
the upload commit form will be stripped so that we do not have invalid
data in the database.

  • Created a review request with history in a Hg repository.
  • Ran unit tests.