Allow fetching directly from promisor remote when cloning

As part of the work on plain HTTP promisor remotes a repo with some demos was created a long time ago.

One of these demos, the "server" demo was recently converted into a script. The demo and the script use a modified version of Git that had a few improvements compared to upstream Git.

One of these improvements was about repacking without large objects. This feature was upstreamed in: !1 (closed)

Another improvement is that when a client clones, it can fetch large objects directly from a plain HTTP promisor remote. This issue is about upstreaming that feature.