Found it myself.
In fact, you do not need to do anything. All you have to do after editing your .hgsub to add your subrepo and clone it is to change the directory to this repository, check the required branch or rev and commit using mercury.
In my case, the whole process was (in the terminal):
$ echo "public/assets/common/js/jQuery-File-Upload = [git]git://github.com/blueimp/jQuery-File-Upload.git" >> .hgsub $ cd public/assets/common/js/ $ git clone git://github.com/blueimp/jQuery-File-Upload.git $ cd jQuery-File-Upload $ git checkout jquery-ui $ hg commit -m"ADDED: jQuery-File-Upload subrepo (jquery branch)"
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