[Merge] ~nteodosio/software-properties:deb822-apt-add into software-properties:ubuntu/master

[Nick Rosbrook]

Review: Approve

Thanks, I think the conservative approach is good here. I think there is still some future work to be done in the future, e.g. the "merge" logic in show_list() doesn't work for deb822 sources. I don't consider that a blocker for this though, because I think it's better to at least have a semi-functional --list.
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https://code.launchpad.net/~nteodosio/software-properties/+git/software-properties/+merge/464417
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