Add a warning on Phabricator revisions when they are modifying a file with 0 coverage

[marco-c]

This could go in the staticanalysis/bot because it uses code coverage data which has already been produced, it doesn't have to parse coverage artifacts and so on like the normal coverage build.
Also, staticanalysis/bot already has all the infrastructure to run stuff when a new Phabricator revision is created and to comment on Phabricator revisions.