Prepare Diffusion for hovercards

Summary:
Move Diffusion to be hovercard-ready, and expand our ability to resolve commit references.

• Link unqualified hashes of 7 characters or more which match a commit.
• Link qualified hashes of 5 characters or more which match a commit.
• Support {...} syntax.

Test Plan: {F33896}

Reviewers: chad, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5121