Diffusion - fix first "old ref" in push log

Summary: This is a fake hash of many 0s which ends up being a bad link. Detect the fake hash and don't print a link. Fixes T6826.

Test Plan: looked at push log and no longer saw a many 0 entry for the first old ref.

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: Korvin, epriestley

Maniphest Tasks: T6826

Differential Revision: https://secure.phabricator.com/D11096