Add support for thin packs

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.

There's no need for this to be a pointer to somewhere else.

Move this one as well, letting us have a single way of setting the
callbacks for the remote, and removing fields from the clone options.

The text progress and update_tips callbacks are already part of the
struct, which was meant to unify the callback setup, but the download
one was left out.

Martin Pool was the original author of the code referenced in the clone
example. Make note that he's given his permission and also give him the
proper credit.

This works by having the indexer watch the return
code of the callback, so will only take effect
on object boundaries.