BUG: diff-{index,files,tree} (and git-gui) do not respect the diff.indentHeuristic config setting

So I have

	diff.indentHeuristic = true

and I noticed that git-gui was not using the heuristic. This is because git-gui uses diff-index, and that does not respect the config setting, even though it supports the --indent-heuristic option.

And it looks like diff-files and diff-tree also have the same problem.

I tried a couple of quick-n-dirty things to fix it in diff-index, without success, and I've run out of git-hacking tame, so all I can do for now is throw out a bug report.

diff-index.c explicitly says "no 'diff' UI options" since 83ad63cfeb ("diff: do not use configuration magic at the core-level", 2006-07-08), so maybe this needs to be fixed in git-gui (and maybe elsewhere), but to me it feels like the diff-foo commands should respect the setting.

(CC'ing Michael Haggerty, who added the heuristic.)

(This is git v2.12.2.)

		M.