pull should fetch everything from a remote

[zombiezen]

While I understand the desire for equivalence between a URL and a remote, the truth is that Git doesn't support this well. For push it makes a great deal of sense to keep a "1 command = 1 ref" rule, but for pull, it's uncommon for the author to know exactly what has changed, and pulling from a URL is infrequent. Especially for #80, having all remote refs up-to-date by just running gg pull myfork can be really useful.