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 Revision: https://secure.phabricator.com/D16713