All the compare tools show diff as plain text.

It would be useful for some files to show diff using their corresponding mime type; for example .xml files should be displayed as text/xml and we would gain syntax highlighting.

This is even more noticeable when using the diff sidebar.

For a versioned file, the diff sidebar will display a little "popup" with the old text which does have syntax highlighting. But if we press the "Open Diff Window" button in that popup we get a plain text diff without any highlighting.