[orbital223]

> Leading comma ensures one line diffs

It does not. It just moves the edge case to a different position: trailing comma has the "issue" when adding an argument to the end of the list while leading comma has it when adding an argument to the beginning.

Also, as pointed out by the other commenter, any decent modern diff tool will make it obvious that the change to the existing line is just the addition of a comma, which makes the difference basically moot.