Unlike existing functions, this produces athin packfile by making use of the fact that only new objects appear in the mempack Object Database.

A thin packfile only contains certain objects, but not its whole closure of references. This makes it suitable for efficiently writing sets of new objects to a local repository, by avoiding many small I/O operations.

This relies onwrite_pack (e.g.git_packbuilder_write_buf) to implement the "recency order" optimization step.

Basic measurements comparing against the writing of individual objects show a speedup during when writing large amounts of content on machines with comparatively slow I/O operations, and little to no change on machines with fast I/O operations.

I have used this inNixOS/nix#11330, in conjunction with#6874