git clone --update ?

I'm working on a build script that pulls source from 12 git repos: http://www.chaosreigns.com/wayland/wayland-build-ubuntu-all.sh

I would like to be able to do the equivalent of:

 rm -rf gtk+; git clone git://git.gnome.org/gtk+ --branch gdk-backend-wayland

Without re-downloading the entire thing. Even if I made any kind of modifications to the repo.

One suggestion I've gotten was;

  git checkout gdk-backend-wayland && git fetch git://git.gnome.org/gtk+ gdk-backend-wayland && git reset --hard FETCH_HEAD && git clean -fxd

Possibly replacing the branch name with HEAD if I want the default branch.

It would be nice if, instead, I could just run something like:

  git clone --update git://git.gnome.org/gtk+ --branch gdk-backend-wayland

Bonus points for working if I have not already cloned it.