Merge branch 'np/types'

Merge branch 'np/types'

* np/types:
  Cleanup check_valid in commit-tree.
  make sure enum object_type is signed
  get rid of lookup_object_type()
  convert object type handling from a string to a number
  formalize typename(), and add its reverse type_from_string()
  sha1_file.c: don't ignore an error condition in sha1_loose_object_info()
  sha1_file.c: cleanup "offset" usage
  sha1_file.c: cleanup hdr usage