Fix typo in DiffusionLowLevelGitRefQuery

Summary: $refs_types is undefined.

Test Plan: None. PhabricatorRepositoryRefCursor::TYPE_REF is not used with DiffusionLowLevelGitRefQuery.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25468