Check if repositories are in sync

[technicalpickles]

There are times when there's a problem reported with the syncing process. I'd like to be able to manually see if the sync happened, by asking repository-sync if an originating repo and destination repo are in sync. I think a quick way would just be a non-git diff of the two checkouts.

Same as related to #31

An API would be nice, but I see that the cloning from a webhook enqueues a job rather than doing it run then and there. I'd be totally cool with a command that can be heroku run.