Check for git before all relevant homesick commands

To close #25 I've added a small method to run before git-related homesick commands, which was [originally suggested](https://github.com/technicalpickles/homesick/issues/25#issuecomment-11175658) by @technicalpickles.

I was originally going to insert this into the git-specific functions (`git_clone`, `git_diff`, `git status`, etc), but it would be more expensive [in scenarios that have multiple git commands](https://github.com/christianbundy/homesick/blob/1e70386f5bafc1ea8d9a27fe5c2b3813e51c40d5/lib/homesick.rb#L51), as well as scenarios where git isn't called until the end of the function (which could have all sorts of side effects).