Tweak some display behaviors for indent indicators

Summary:
Ref T13161.

• 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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13161

Differential Revision: https://secure.phabricator.com/D20195