Summary:
Fixes T13468. See that task for discussion. The older source-rendering code mixes "line number" / "1-based" lists with "block number" / "0-based" lists and then has other bugs which cancel this out.

For block-based diffs, build an explicit block-based mask with only block numbers. This sort of sidesteps the whole issue.

Test Plan: Viewed the diff with the original reproduction case, plus various other block-based diffs, including one-block image diffs, in unified and side-by-side mode. Didn't spot any oddities.

Maniphest Tasks: T13468

Differential Revision: https://secure.phabricator.com/D20959