Move from unsigned char[20] to struct object_id continues.

* bc/object-id:
  match-trees: convert several leaf functions to use struct object_id
  tree-walk: convert tree_entry_extract() to use struct object_id
  struct name_entry: use struct object_id instead of unsigned char sha1[20]
  match-trees: convert shift_tree() and shift_tree_by() to use object_id
  test-match-trees: convert to use struct object_id
  sha1-name: introduce a get_oid() function