message: Fix call to append on bytes
  Summary:
  Introduced in a recent refactoring, which changed a call to `operator.concat`
  with a call to `append`, which does not exist on bytestrings or strings.  I
  think the equivalent code should use `+=` instead.
diff: @@ -34,7 +34,7 @@ def parse_hprof_dump(instream):
  break
  if byte == b"\x00":
  break
  - tag.append(byte)
  + tag += byte
  tag = tag.decode("utf-8") # UTF8 should be close enough to modified UTF8.
  big_endian_unsigned_4byte_integer = struct.Struct(b">I")
  