[Merge] ~mwhudson/livecd-rootfs/+git/livecd-rootfs:desktop-source-catalogs into livecd-rootfs:ubuntu/master

[Sebastien Bacher]

Review: Approve

The checkout-translations-branch call is now copied in different cases, could make sense to factor in a function so if we ever need to change the url or something it's one place to update? Otherwise the change seems fine to me, thanks for working on it!
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https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/408449
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