What if somebody had poller state with non-bare branch names?

If we are going to try to be smart, we should be smart, and canonicallize names. (It appears that `ls-remote` will do that for us). That will deal with anything, going forward.

I'm not sure how best to deal with existing state. One option would be to version `lastRev`. i.e. Have `lastRev` and `lastRev-v2`, with the latter having only canonicalized names. And to covert the former to the latter, run `ls-remote` (which will convert branches that still exist).