git clean on pre-existing git repo discovery by maybe source

[sdboyer]

I'm seeing an increasing number of reports where some corruption is happening in the source cache. tracking down the source of the corruption is generally nigh-impossible (i tend to assume it results from a bad exit from a Ctrl-C, but it doesn't matter - #553 might also be a cause?)

one simple thing we can do to help address issues would be to verify cleanliness of on-disk repository working tree in the maybeSources, upon discovering that one already lives on disk. this has the advantage of being a one-time operation, rather than being something that we need to incorporate into the FSM for correct repo state management.