allow selecting a non-existent branch as upstream
In the case of `magit-push-current-to-upstream' this fixes a regression. In the case of `magit-set-branch*merge/remote' this is a regular change, though one could argue that the name always suggested that one could use a non-existent branch. Improve `magit-set-branch*merge/remote' for non-interactive use, and use it in `magit-push-current-to-upstream'.
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