Warn about dirty directory when running 'rbt post' with Mercurial repository

As with Git, the Mercurial backend only posts changesets and ignores
uncommitted changes. Warn users about this, since they may expect to be able to
post the same output as 'hg diff'.

Testing Done:
Ran 'rbt post' both with and without uncommitted changes.
I'm not sure if we want the '--deleted' flag included, since it's files that
are tracked but have been deleted - e.g. using 'rm file' instead of 'hg rm
file'.

Reviewed at https://reviews.reviewboard.org/r/6082/