Fix up some bad test data for diffviewer tests.
Review Request #12696 — Created Oct. 24, 2022 and submitted
We had a couple tests in the diffviewer module that were technically
including incorrect index lines for the diff being applied. We hadn't
noticed this because GNU patch is pretty good at applying patches even
when they don't quite match up perfectly, but Apple patch (which we're
not actually supporting) exposed these. This change fixes that up, just
for the sake of correctness.
Ran unit tests.