R1:c20fd5830374

Add a Diffusion repository remarkup rule

Summary: Currently we markup `rXabcd`, but not `rX` on its own. Mark these up as repository object names.

Test Plan: Typed `rPOEMS`, `rPOEMS1`, `rPOEMS139893189`, etc.

Reviewers: btrahan, dctrwatson

Reviewed By: btrahan

CC: aran, poop

Differential Revision: https://secure.phabricator.com/D7859