cr: limit the number of canceled we can get
cr: limit the number of canceled we can get

Since we didn't set `lockNextTime` until after fetching all MDs, we
might let the size of the branch grow very large.

Also avoid setting the CR summary for just pending local squashes.

Issue: KBFS-1653

keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client