repo: Fix static delta progress display

repo: Fix static delta progress display

There were a few bugs here.

- We need to keep track of the size of the delta parts we've already processed,
  in order to make progress reliable at all in the face of interruptions.  Add
  a new `fetched-delta-part-size` async progress variable for this.
- The total before disregarded what we'd already downloaded, which was confusing.
  Now, a progress percentage is `fetched/total`.
- Correctly handle "unknown bytes/sec" in the progress display.

However, to be fully correct we need to show the fallback objects too. That
would require tracking in the pull code when we fetch an object as a fallback
versus "normally". This would be simpler really if we could assume in a run we
were *only* processing a delta, but currently we don't do that.

Related: https://github.com/ostreedev/ostree/issues/475

Closes: #678
Approved by: giuseppe