Remove hard-coding of diff line height

Summary: Fixes T10959. This is the smallest/simplest fix that I could come up with, and I wasn't able to break it. Basically, I removed "line-height" and then adjusted other rules until the defaults looked reasonable again.

Test Plan:
Here's 24px / 48px impact or something like it:

Here's normal stuff working properly without weird artifacts on the highlighting:

Also tested Firefox and Chrome and got similar results.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: wxm20073527, Korvin

Maniphest Tasks: T10959

Differential Revision: https://secure.phabricator.com/D15905