sideways scroll with touchpad in diff view

[philz]

Monaco supports touchpad sideways scrolling, but it doesn't quite work right in diff view.

See also commit e0860935

I'm still seeing two bugs:

If you click into the diff the first time on some line, especially
if it's the second diff block, Monaco jumps to a different area. This
is kind of terrible and annoying.

My touchpad should let me scroll right/left as well as vertical, but
I'm only getting vertical scrolling in this mode.