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.
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https://www.mercurial-scm.org/wiki/ContributingChanges
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https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).