Setting for updateDiffOnSave

With this enabled (the default), saving a file will copy the main pane
value to the diff pane. This means you can see what changes you are
making prior to a save and also can undo changes in diff pane to see
save points and how they differ to the main pane.
updateDiffOnSave available in JS

Used in if statement to determine if we should do this or not
Apply new JS value to updateDiffOnSave flag