Remote revamp (director's cut)

The callback-based method of listing remote references dates back to the
beginning of the network code's lifetime, when we didn't know any
better.

We need to keep the list around for update_tips() after disconnect() so
let's make use of this to simply give the user a pointer to the array so
they can write straightforward code instead of having to go through a
callback.

Fix examples to make the important stuff more obvious

I'm not too happy about manually inserting < and > but those get
output as html tags otherwise.

It was there to keep it apart from the one which read in from a file on
disk. This other indexer does not exist anymore, so there is no need for
anything other than git_indexer to refer to it.

While here, rename _add() function to _append() and _finalize() to
_commit(). The former change is cosmetic, while the latter avoids
talking about "finalizing", which OO languages use to mean something
completely different.

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.