Here goes:
$ cat .gitmodules
[submodule "utils/external/firepython"]
path = utils/external/firepython
url = git://github.com/darwin/firepython.git
[submodule "utils/external/textile"]
path = utils/external/textile
url = git://github.com/jsamsa/python-textile.git
While this was still a Git repo, I needed to run git submodule init, after which some magic happens. Since I've now converted the repo to Mercurial (using hgext.git extension), I don't know what to do. Is there an equivalent process (I need those 2 Git modules in my Mercurial repo)?
Mercurial supports subrepositories of different kinds: Mercurial, Subversion, and Git. So you can create a .hgsub file with
utils/external/firepython = [git]git://github.com/darwin/firepython.git
utils/external/textile = [git]git://github.com/jsamsa/python-textile.git
and that will inform Mercurial to make a clone of your Git repositories when the Mercurial repository is cloned. You need to make the Git clones yourself the first time, or copy them from somewhere else on your disk:
$ git clone git://github.com/darwin/firepython.git utils/external/firepython
$ git clone git://github.com/jsamsa/python-textile.git utils/external/textile
$ hg add .hgsub
$ hg commit -m 'Added Git subrepositories'
You will then note that Mercurial has added a .hgsubstate file to your repository where it stores information about the Git subrepositories. This file is needed so that Mercurial knows which revision to checkout from your subrepositories when you make a new Mercurial clone.
A colleague of mine has written a subrepository guide that you might find useful.