Optionally disable git_prompt_info with git config option for very large repos

This adds a check to git_prompt_info for the git config option `oh-my-zsh.hide-status` before doing anything.

Having repo status in the prompt for very large repos becomes unmanageable and `git status` takes too long to return if it is executed before every PROMPT is shown.

This allows git_prompt_info to be enabled by default but disabled on a repo by repo basis if the flag is set to `1`.

This is nothing more than an updated version of https://github.com/robbyrussell/oh-my-zsh/pull/1772 to bring it up to date with the current layout of lib/git.zsh