Highlight inline diff context in HTML mail

Summary:
Ref T10694. Ref T9790. When generating inline diff context, highlight it and then mangle the highlighted output into style="..." so it works in HTML.

Also try to tighten up some spacing/formatting stuff.

Test Plan:
Got some output in this vein:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9790, T10694

Differential Revision: https://secure.phabricator.com/D15852