R1:4e831e786e7a

Fix Phriction document move on to existing document placeholder

Summary:
Looks like the logic was there already but some minor parts were missing.
Fixes T8082.

Test Plan:
- Create document `/w/foo`
- Delete document `/w/foo`
- Create document `/w/bar`
- Move document `/w/bar` for `/w/foo`
No error was displayed and document `/w/bar` was moved to `/w/foo`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T8082

Differential…