git_prompt_status() slows down zsh with big git repositories

When you use a theme which uses the git plugin (like fishy), the prompt generation is *very slow* and sometimes it takes 2-3s to display the prompt. Even if you set the option "oh-my-zsh.hide-status" thought git config, it's slow. After some tests, I found that "git status" is the problem. It is actually used in git_prompt_status() to get the status of the current working tree and update the prompt.

I propose to don't update the prompt when the option "oh-my-zsh.hide-status"  is set, it would be very useful for important git repositories (like qtbase on gitorious)