-g sets up git repo. -f forces even if already there. I found these useful.
Closes Homebrew/homebrew#13689. Signed-off-by: Max Howell <mxcl@me.com> There are subtle distinctions between writable? and writable_real? we don't understand precisely why we need this, but it fixes the bugs :/
Fixes Homebrew/homebrew#10829.