Default to rich diffs for SVG files #1599

When SVG files are edited, usually the rich diff (rendered image) is more useful/desirable than textual diff.

This is particularly useful when such files are edited in an editor rather than by hand, since the changes are usually extensive enough to go over the size limit for showing the diff automatically, so neither the code nor the rich diff are shown by default (the "Load diff" link is shown instead). See for example pretty much any PR at icon repos, such as https://github.com/elementary/icons/pulls.