Before this patch, we use a combination of mode and labels to
decide which marker style to use. That's mostly my own fault because I
was the one who added the mergediff as a "mode" instead of adding a
separate argument for marker style. Still, let's make these arguments
more orthogonal by ignoring the number of labels when deciding which
marker style to use.
This refactoring makes it a little clearer in the simplemerge
*extension* that passing more than 2 --label arguments changes the