Ref T13249.
- When a line has only increased in indent depth, don't red-fill highlight the left side of the diff. Since reading a diff mostly involves focusing on the right side, indent depth changes are generally visible enough without this extra hint. The extra hint can become distracting in cases where there is a large block of indent depth changes.
- Move the markers slightly to the left, to align them with the gutter.
- Make them slightly opaque so they're a little less prominent.
