root_metadata: deepCopy must also copy the cached block changes
root_metadata: deepCopy must also copy the cached block changes

Otherwise, when the journal converts MDs into unmerged MDs, we might
lose the old block change pointer in the cached copy.

Issue: KBFS-1637

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