The new indexer needs to be able to bypass any kind of buffering, as
it's trying to map data that it has just written to disk.

This will allow us to index a packfile as soon as we receive it from
the network as well as storing it with its final name so we don't need
to pass temporary file names around.

Resolve any lingering deltas, write out the index file and rename the
packfile.

This changes the git_remote_download() API, but the existing one is
silly, so you don't get to complain.

The new API allows to know how much data has been downloaded, how many
objects we expect in total and how many we've processed.

Trying to send every single line immediately won't give us any speed
improvement and duplicates the code we need for other transports. Make
the git transport use the same buffer functions as HTTP.

This allows us to give updates on how it's doing

In the case that walk->one is NULL, we know that we have no positive
references, so we already know that the revwalk is over.