Tweak some display behaviors for indent indicators
- Don't show ">>" when the line indentation changed but the text also changed, this is just "the line changed".
- The indicator seems a little cleaner if we just reuse the existing "bright" colors, which already have colorblind colors anyway.
Test Plan: Got slightly better rendering for some diffs locally.
Reviewed By: amckinley
Maniphest Tasks: T13161
Differential Revision: https://secure.phabricator.com/D20195