Fix two "msort()" vs "msortv()" issues in "arc land"

Summary: See https://github.com/phacility/arcanist/pull/242. Ref T13303. A little more fallout turned up in arc.

Test Plan: grep msort(

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13303

Differential Revision: https://secure.phabricator.com/D20605