FR: horizontal scrollbar for n-way diff

[ilyagr]

When doing an n-way diff for n>3, the screen becomes very cramped.

It'd be nice if diffuse could show just ~2.5 files on one screen, but there was a horizontal scrollbar to move between all files.

[Yopai]

I just used diffuse to compare 12 files ... Fortunately I have 3 screens, but it would be simpler with an horizontal scrollbar