Fix an issue in Owners where a transaction change could show too many effects

Summary:
Fixes T13324. Ref PHI1288. Currently, if you edit an Owners package that has some paths with no trailing slashes (like README.md) so their internal names and display names differ (/README.md display, vs /README.md/ internal), the "Show Details" in the transaction log shows the path as re-normalized even if you didn't touch it.

Instead, be more careful about handling display paths vs internal paths.

(This code on the whole is significantly less clear than it probably could be, but this issue is so minor that I'm hesitant to start ripping things out.)

Test Plan:

• In a package with some paths like /src/ and some paths like /src:
• Added new paths.
• Removed paths.
• Changed paths from /src/ to /src.
• Changed paths from /src to /src/.

In all cases, the "paths" list and the transaction record identically reflected the edit in the way I expected them to.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13324

Differential Revision: https://secure.phabricator.com/D20596