local: create pack with multiple threads

local: create pack with multiple threads

The default behaviour for the packbuilder is to perform the work in a
single thread, which is fine for the public API, but we currently have
no way for a user to determine the number of threads to use when
creating the packfile, which makes our clone behaviour over the
filesystem quite a bit slower than what git offers.

This is a very particular scenario, in which we avoid spawning git by
being ourselves the server-side, so it's probably ok to auto-set the
threading, as the upload-pack process would do if we were talking to
git.