Implement parse_revision_spec for mercurial.
This change implements revision parsing for mercurial. This currently handles

all of the functionality that's currently supported, handling zero, one, or two

revisions. Parent diffs are currently not supported, for a couple different

reasons. Parent diffs with zero arguments are easy, but the --parent argument

is currently used as a way of specifying the remote branch. For one or two

arguments, we need to detect if the base revision is outgoing, but I haven't

entirely figured out how to do that yet.

Ran unit tests.