Make RelationCounterField's rel_field_name default to None

Review Request #6642 — Created Nov. 24, 2014 and submitted — Latest diff uploaded

Information

Djblets
master
8678880...

Reviewers

This allows django-evolution to add, change, and remove instances of
RelationCounterField.

Ran unit tests.

Ran an evolution that created a RelationCounterField.