Use a stricter regexp in Diffusion remarkup

Summary:
Just minor bookkeeping, but the current regexp is too liberal and
will match things which can't possibly be revision hashes.

Test Plan:
Typed things which should and shouldn't be revision links, they
got handled properly.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran
Differential Revision: 185