R1:b05b9f240865

Link hashes in Remarkup commit links

Summary: Also generalize the code a bit.

Test Plan:
rPF57bc7f#inline-874
rPF57bc7f#874

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4168