1. Enable "Display right margin at column" in "Edit" -> "Preferences..." -> "Text Editor" -> "Display".
   Editors will now show a vertical line to indicate the desired right margin of text.
2. Open a file from a git repository.
   The issue is the same when diffing two plain files. Git just makes it easier to demonstrate.
3. Do some changes in the file and save it.
4. From the menu, select "Tools" -> "Git" -> "Current File" -> "Diff of <filename>".
   A diff editor shows the changes in the file. Removed lines are shown on a red background, added lines on green. Both of these backgrounds cover the vertical line so the user can't see the right margin where it would be most useful:
   

The right margin line should also be visible in changed lines.