Adds force_checkout option to git state.

I clone the homebrew repo to /usr/local/homebrew and on occasion changes will occur to the one or more files in the repo during the typical usage of brew. Most times it is just permissions, but no matter the changes, if I'm running state.highstate I'm expecting the repo that is managed via git.latest to be up to date after the salt run if I've passed enough options to allow git.latest to be destructive.