Make it easier to restore the commit graph after sorting by a column

[rowanj]

From #251

[Uncommon]

I always thought it was easy enough. Click the sorted column to reverse, then again to go back to the graph.

[sugarmo]

👍 for @Uncommon's great tips.

[ljharb]

Per the discussion on #251, a three-state sortable column header is not something I've ever seen before, and I don't find it intuitive. I'm glad to be informed about it, but it'd be ideal to have something that explicitly calls it out, even if it's just a menu item.

[voxpelli]

I find it intuitive, but it's evidently a bit hard to discover, so something should probably be added but the existing functionality can remain.