R1:fb4b9bc2fc61

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…