infinitepush: skip fastheadsdiscovery heuristic
remotenames extension has a heuristic that speeds up discovery. Unfortunately
it may result in sending public commits to infinitepush server. This diff
intentionally disables this heuristic for infinitepush backups.
Test Plan: Run tests
Reviewers: #fbhgext, mbthomas
Reviewed By: #fbhgext, mbthomas
Differential Revision: https://phab.mercurial-scm.org/D1505