I think you want to know the replacement of svn:external in git. I'm also searching this by myself. After playing around with git for my projects, I think I will not need it anymore. For a case that I'm using module from other project, I simply keep it in other separated dir.
For your example, I will clone jquery to source/jquery folder. Then when this is updated, I will simply copy all of it content to my application, such as source/app/module/jquery, then commit the source/app. Since I will not touch the source/app/module/jquery, I have no need to check what have been updated there, but simply just overwrite it with the latest update from source/jquery folder.
I know this will have extra efforts than automatic svn:external, but I think git offer more feature that svn didn't have, that weight more that a single svn:external feature. That's why I switch all my project from svn to git and never look back, ever.
I also thinking of using hooks in the source/app to do the copy for me, but haven't got any time to write one. Maybe you can experiment this one.