Fix the wrapping/padding behavior of Remarkup code block headers more thoroughly?

Summary: Ref T13118. The first fix there fixed Safari, but made Chrome weird. Try this?

Test Plan: Viewed a code block with name=... in Safari, Firefox and Chrome and saw consistent display without weird wrappping.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13118

Differential Revision: https://secure.phabricator.com/D19319