cr: proactively check for cancels during resolution completion
cr: proactively check for cancels during resolution completion

Branch squashes can take a while, so if they're canceled it's good to
know ASAP to avoid wasting work.

Issue: KBFS-1653

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