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

src/infrastructure/markup/remarkup/markuprule/diffusion/PhabricatorRemarkupRuleDiffusion.php

@@ -24,7 +24,7 @@ class PhabricatorRemarkupRuleDiffusion
 
   public function apply($text) {
     return preg_replace_callback(
-      '@\br([A-Z]+[a-z0-9]+)\b@',
+      '@\br([A-Z]+[a-f0-9]+)\b@',
       array($this, 'markupDiffusionLink'),
       $text);
   }