Unlike existing functions, this produces a _thin_ 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 on write_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.
