Write a thin packfile with the objects in the memory store.

A thin packfile is a packfile that does not contain its transitive closure of references. This is useful for efficiently distributing additions to a repository over the network, but also finds use in the efficient bulk addition of objects to a repository, locally.

This operation performs the (shallow) insert operations into the git_packbuilder, but does not write the packfile to disk; see git_packbuilder_write_buf.

It also does not reset the in-memory object database; see git_mempack_reset.