Fix a "setExternalURI()" fatal while browsing directories with submodules

Summary:
Ref T13595. See that task for discussion.

D21511 renamed the iteration variable here (previously "$path") but did not rename this use of it.

Test Plan:

• In Diffusion, browsed a directory with a submodule.
  • Before: "setExternalURI()" fatal in conduit call.
  • After: directory listing including submodule.

Maniphest Tasks: T13595

Differential Revision: https://secure.phabricator.com/D21520