Here\'s part of the contents of my .gitmodules
file:
[submodule \"src/static_management\"]
path = src/static_management
url = gi
According to the answer from Dave James Miller I can confirm that it worked for me. The important thing here was to commit the subprojects commit ID. Just to have the entry in .gitmodules was not enough.
Here is an appropriate commit:
https://github.com/dirkaholic/vagrant-php-dev-box/commit/d5f4c40bdbd80eefbb5ac6029823733f591435ae