Don't try to commit unchanged specs on push

If multiple podspecs exist in a directory, and I run 'pod push my-repo' when some of those podspecs haven't changed, the push raises an exception when trying to run 'git commit' with no changes.

I propose we simply notify that the file is unchanged and allow the process to continue, rather than exiting the process.

This PR simply checks to see if the `git diff` for the file actually has a change before it attempts to add and commit.