Only revert commits in the same repository
Summary: Fixes T8215. When adding a revert edge, only consider commits within the same repository.
Test Plan: Imported the arcanist repository three times and inspected the reverts relationships on rARC490984936bb7ddfc589ef3d376ec2ac2365f070d.
Reviewers: hach-que, epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T8215
Differential Revision: https://secure.phabricator.com/D12875