If the server is more up-to-date, your push will get rejected and a Growl-style notification will reflect that. Simple. I could see adding an option automating

If the server is more up-to-date, your push will get rejected and a Growl-style notification will reflect that. Simple. I could see adding an option automating pulling before pushing, in which case a local conflict could occur. In that case, restoring the local HEAD to what it was before the pull would probably be the best behavior.