Correctly combine hash codes for repeated enums. (#556)

The existing logic in `hashField` is overriding the previously computed
hash codes, which could include values from other fields within the
message. Any message with a repeated enum field with a (relatively) high
tag number will generate a poor hash code which doesn't reflect other
fields.

Co-authored-by: Loren Van Spronsen <lorenvs@google.com>