TextWrangler already installs the command line utility "edit" so the hardest pard is done, if you did not install it for some reason, then just go to "TextWrangler --> Install Command Line Tools..." and you should be all set with this part.
To make Git play nice (or should I rather say TextWrangler play nice with Git), we need to properly set it up as the default editor for Git. And luckily it's as easy as
git config --global core.editor "edit --wait --resume "$@""