Merge branch 'ap/packfile-promisor-object-optim' into jch

The code path that enumerates promisor objects have been optimized
to skip pointlessly parsing blob objects.

* ap/packfile-promisor-object-optim:
  packfile: skip hash checks in add_promisor_object()
  object: apply skip_hash and discard_tree optimizations to unknown blobs too

diff --cc packfile.c
Simple merge