Fix an issue where paths could bleed across repos in Owners

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8320

Differential Revision: https://secure.phabricator.com/D14022