The old code was incorrect and also broken. It passed the tests by accident.
The new algorithm is:
        1) Sort the fields in order of names.
        2) For all fields with the same name, sort in increasing depth.
        3) Choose the single field with shortest depth.
If any of the fields of a given name has a tag, do the above using
tagged fields of that name only.
Fixes #5245.

R=iant
CC=golang-dev
https://golang.org/cl/8583044