(stable) Fix a possible fatal on the first push to a cluster repository

Summary:
Fixes T11020. I think this resolves things -- $new_version (set above) should be used, not $new_log directly.

Specifically, we would get into trouble if the initial push failed for some reason (working copy not initialized yet, commit hook rejected, etc).

Test Plan: Made a bad push to a new repository. Saw it freeze before the patch and succeed afterwards.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11020

Differential Revision: https://secure.phabricator.com/D15969