Fix Diffusion fatal with symbol highlighting

Summary: This returns a list of PHIDs, not objects which need to be pulled.

Test Plan:
Repro'd fatal locally, verified it was fixed.

Repro steps are:

• Create an arc project associated with a repository, with indexed language(s) and subprojects.
• View a file in that repository.

Reviewers: nh, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3038