spartan: make actual changeset entries have backgrounds on /graph This patch removes a separate container for backgrounds only and puts backgrounds on changeset elements themselves. This makes it so that JS code doesn't need to create background elements separately anymore. There's a bit of manipulating positions of elements: every changeset entry has a "fg" element that gets a higher z-index than <canvas> element. This prioritizes text information, so that even if it somehow gets close to the graph drawn on <canvas> (which has z-index of 5), it'll still be on top of it. This can happen if commit message has a long first line and browser window is narrow, for example.