Fix an issue where paths could bleed across repos in Owners
Ref T8320. I missed this a while ago and then it came to me in a dream.
Only consider paths in the same repo when looking at ownership.
(I think this is rarely reachable in practice.)
Test Plan: Verified that files and commits still listed ownership properly.
Reviewed By: chad
Maniphest Tasks: T8320
Differential Revision: https://secure.phabricator.com/D14022