bup already splits files and does rolling deltas, so there is no reason to
use chunking here.
The new API made it easier to add progress support for storeKey, so that's
done. Unfortunately, bup-split still outputs its own progress with -q,
so a little ugly, but not too bad.
Made dropping remove the branch for an object, for two reasons:
1. The new API calls removeKey to roll back a storeKey when the content
changed unexpectedly.
2. So that testremote will be happy.
Also, fixed a bug that caused a crash when removing the branch for an
object in rollback.
b261df735d