examples: show used local objects in fetch

Show how many local objects were used to fix the thin pack in our fetch
example.
indexer: include the delta stats

The user is unable to derive the number of deltas in the pack, as that
would require them to capture the stats exactly in the moment between
download and final processing, which is abstracted away in the fetch.

Capture these numbers for the user and expose them in the progress
struct. The clone and fetch examples now also present this information
to the user.