R1:03a1deba23f6

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:

{F1310445}

{F1310446}

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

{F1310447}

Also tested Firefox and Chrome and got similar results.