Treat missing branch like missing checkout with --disable-updates

[chergert]

It's possible that a branch has changed in a manifest for which there is already a local git checkout in flatpak-builder state. --disable-updates will not download the new branch because there is already a checkout.

flatpak-builder should treat a missing branch the same as a missing checkout when --disable-updates is specified.

[alexlarsson]

Yeah, that would be good. It needs some work to figure out if the target ref exists though.