Merge branch 'bw/object-id'

Conversion from uchar[20] to struct object_id continues.

* bw/object-id:
  receive-pack: don't access hash of NULL object_id pointer
  notes: don't access hash of NULL object_id pointer
  tree-diff: don't access hash of NULL object_id pointer

diff --cc builtin/notes.c
Simple merge

diff --cc builtin/receive-pack.c
Simple merge

diff --cc tree-diff.c
Simple merge