Fix an issue where entering the same Owners path for two repositories would incorrectly de-dupe the path

Summary:
Ref T13130. See https://discourse.phabricator-community.org/t/unable-to-create-owners-package-with-same-path-in-multiple-repositories/1400/1.

When you edit paths in Owners, we deduplicate similar paths, like /x/y and /x/y/. However, this logic currently only examines the paths, and incorrectly deduplicates the same path in different repositories.

Instead, consider the repository before deduplicating.

Test Plan:

• Edited an Owners package and added the path "/" in two different repositories.
• Before: only one surived the edit.
• After: both survived.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13130

Differential Revision: https://secure.phabricator.com/D19420