Fix Mercurial unable to authenticate with HTTP when pulling

Summary: As described in T7959, it looks like Diffusion does not provide Mercurial the required HTTP credentials when pulling from an external repository.

Test Plan: Add an external Mercurial repository to Diffusion, that requires HTTP authentication. A private BitBucket repository for example.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Projects: #mercurial, #diffusion

Differential Revision: https://secure.phabricator.com/D14092