Handle case when Homebrew checkout is shallow

Using hub to fork Homebrew and push a branch we've just created to this
fork will be rejected by GitHub because the push is coming from a
shallow repository. Ensure that Homebrew is unshallowed before that.

[ci skip]

script/brew-publish

@@ -31,7 +31,7 @@ fi
 
 pushd "$(brew --prefix)"
 
-git fetch -q origin master
+git fetch -q --unshallow origin master
 
 branch="${brew_name}-${version}"
 git checkout -q -B "$branch" origin/master