I ran into a small issue after updating XCode via the MacApp Store. Basically installing Xcode now removes the old /Developer directory from the root of your hard drive and it now lives inside of the Xcode.app directory.
If you use the built in OS X merge tool for fixing merge issues in git repose then the following git command will break and not work.
$> git mergetool -y
In order for it to work again you need to run the following command in terminal.
$> sudo /usr/bin/xcode-select -switch /Applications/Xcode.app/Contents/Developer
Hope this little tip helps.