folder_branch_ops: notifyOneOpLocked should take ReadOnly md
folder_branch_ops: notifyOneOpLocked should take ReadOnly md

Since it doesn't depend on the mdID or other immutable fields.

Suggested by akalin-keybase.

Issue: #944

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