Does Mac OSX Mountain Lion autosave/versions interact badly with git?
Short answer: don't worry about it.

The versions data is kept separate from the file itself, which is why you can just email a file to someone on a different OS and not worry about the version info mucking things up.

Unless you are checking in the hidden folders where OS X stores temporary files and versions, git will never know if a file is being versioned or saved by the OS.