dub run replacement for ~branch

[ntrel]

If using a git repository URL is going to fully replace ~branch, we would also need a way to specify a git branch for dub run. E.g. currently this works (though master is only fetched the first time - #131):

dub run dlang-tour@~master -- --lang-dir .

[Geod24]

This version of dub run hits the registry. I would imagine dub run git+https://github.com/dlang/dlang-tour@~master would be the "proper" replacement although I'm not sure we want to support this.