walk: make the sorting mode optional

Since the libgit2 has a default sorting method, which we also mention as
default in the documentation, there is no particular need to make the
user choose a sorting method when the order does not matter. We use
sorting NONE in that case.
repository: add listall_reference_objects() method

This allows for efficient reading of many references and their targets,
without incurring the overhead of lookup_reference() (which stats for
a loose ref and then reads packed-refs) which can be expensive on NFS
with thousands of refs.