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.