Merge branch 'jk/alloc-commit-id-maint' into maint

Merge branch 'jk/alloc-commit-id-maint' into maint

* jk/alloc-commit-id-maint:
  diff-tree: avoid lookup_unknown_object
  object_as_type: set commit index
  alloc: factor out commit index
  add object_as_type helper for casting objects
  parse_object_buffer: do not set object type
  move setting of object->type to alloc_* functions
  alloc: write out allocator definitions
  alloc.c: remove the alloc_raw_commit_node() function