Fix fetch() on git < 1.7.3

Apparently these versions of git are broken for `git fetch $url`, so we have to
resort to `git fetch origin` and then just pray that it hasn't changed in the
meantime.
auto merge of #101 : alexcrichton/cargo/fix-git-1.7, r=wycats

Apparently these versions of git are broken for `git fetch $url`, so we have to
resort to `git fetch origin` and then just pray that it hasn't changed in the
meantime.