Diffusion - missed a "dont load diffusion request" in the code serving pathway

Summary: Fixes T6939.

Test Plan: From the task, visited a URI like http://code.example.com/diffusion/REPO/repo.git/info/refs?service=git-upload-pack. Before the patch, I got an error and post patch I get a nice login prompt to provide credentials to the repository, as expected based on my confguration

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6939

Differential Revision: https://secure.phabricator.com/D11348