R1:a3ebaac0f026

Tweak the visual style of the ">>" / "<<" depth change indicators slightly

Summary:
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…