The first arg to fetch & pull is now a remote. In addition, the default
remote if you don't specify is now the tracking remote as in `git pull`
if it exists, and origin if that's not set. This makes it more consistent
with the underlying git workflow especially in triangular fork setups.
Now that we're not pretending to download objects we already have, we wouldn't
check out objects missing in the working copy but which were already present.
It's better to do exactly what it says on the tin anyway, identical to
separate fetch & pull.
