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 the gutter.
• Make them slightly opaque so they're a little less prominent.

Test Plan: See screenshots.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13249

Differential Revision: https://secure.phabricator.com/D20251