$ git submodule update
You need to run this command from the toplevel of the working tree.
You can update only a specific repo if you'd like. In which case it would be possible to limit the submodule command to only submodules existing in the current directory.
Yes, it would need to scale back up to the .gitmodules file, but it is still possible.
In other words, git knows "exactly where that is" by searching parent directories until it sees a ".git". Unfortunately, submodules also have ".git" in them since they're full git repositories themselves.
git submodules are not the nicest part of git, unfortunately.
For those also annoyed, I use this git alias, which does what I want:
sup = !sh -c 'cd `git rev-parse --show-toplevel` && git submodule update --init' -I mean, christ, I fetch and merge as separate steps.