R1:79154455437b

Fix two issues with Differential updates and Owners

Summary:
Ref T13114.

- Followup fix for D19267, which didn't work correctly with //new// revision creation.
- Followup fix for changes in T11015. Some of the querying logic was still handling "/x.y" and "/x.y/" differently. Instead, normalize consistently to "/x.y/"

Test Plan:
- Created a new revision cleanly.
- Created a package owning only a `example.txt` file and saw Differential find it as an owning package in the table of contents.