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# get the submodule initially
git submodule add ssh://bla submodule_dir
git submodule init

# time passes, submodule upstream is updated
# and you now want to update

# change to the submodule directory
cd submodule_dir

# checkout desired branch
git checkout master

# update
git pull

# get back to your project root
cd ..

# now the submodules are in the state you want, so
git commit -am "Pulled down update to submodule_dir"