R1:ccf28a81121e

Fix an issue where the last line of block-based diffs could be incorrectly hidden

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…