Optimize matching regexps in Herald rules

Summary:
We spend 6.37 s in this condition on a big diff.
By adding the 'S' flag, the time is down to 2.15 s.

Test Plan: DifferentialRevisionEditor::newRevisionFromConduitWithDiff()

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3284