home: markdown formatting for changelogs in git

I discovered `guix pull --news` recently and realised I barely ever keep
up with nixpkgs's change logs, so here's a thing which might help to
change that. It's actually a little more general than I thought it'd
need to be — turns out that people do change old release notes items in
nixpkgs (mostly to update links), so it can format deletions as well.

(some vague inspiration was given by https://github.com/netj/markdown-diff)

It's a little cursed, but then so is using `lowdown` at all I'd say …

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