R1:344a2e39bed7

In Jupyter notebooks, apply intraline diffing to source code lines

Summary: Ref T13425. When we render a diff between two source lines, highlight intraline changes.

Test Plan: Viewed a Jupyter notebook with a code diff in it, saw the changed subsequences in the line highlighted.

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20851