### add new plugin with git submodule
# example:
# git submodule add http://github.com/tpope/vim-fugitive.git bundle/fugitive
# git add .
# git commit -m "Install Fugitive.vim bundle as a submodule."

#### upgrading a plugin bundle
# cd ~/.vim/bundle/fugitive
# git pull origin master

### Upgrading all bundled plugins
# git submodule foreach git pull origin master


You can reference this page.
http://vimcasts.org/episodes/synchronizing-plugins-with-git-submodules-and-pathogen/