When comparing diffs with five or more lines, show the 'move'-button
markers even when using range connections in order to assist the user in
finding the move buttons, which only appear when hovering over them.
For smaller diffs the marker is not shown to clutter the UI less as the
markers are then relatively easy to find in the middle of the range.

Follow-up on
- https://github.com/eclipse-platform/eclipse.platform/pull/2565