Fix commit parsing in owners tool after bug introduced in 30ae22bfcff71fa3f14e38fd3cd597448df501c3

Summary:
A bug was introduced in
rP30ae22bfcff71fa3f14e38fd3cd597448df501c3 which caused commits to start
being parsed as if the repository callsign was empty. This caused errors
and problems and much unhappiness. Example error:

EXCEPTION: (Exception) No such repository ''. at [.../phabricator/src/applications/diffusion/request/base/DiffusionRequest.php:130]

Test Plan: Saw that commit parsing was breaking. Applied fix. Saw that commits parsed again.

Reviewers: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1974