Add "Show combined view" button

[arai-a]

In the combined view, there are "Show single section" buttons to switch to the specific section. I sometimes want the opposite. Of course the same can be done by selecting "Combined view" in the Sections dropdown menu, but if there are many changes, the dropdown can be long, and scrolling and selecting it is tedious.

One of the reasons this is necessary is that the link to other section doesn't open the diff, but opens the snapshot. So, addressing it may reduce the necessity