[yocto] How to use Git sub-module fetcher (gitsm://) in a recipe?

[Glenn Schmottlach]

I noticed a "new" fetcher recently that appears to support fetching
Git sub-modules from within a Git project. Unfortunately, I cannot
find any examples of how it's used from within a recipe.

There is a GitHub project I'd like my fetcher to clone that contains a
submodule. Obviously I'd like the fetcher to grab both the main
project and recursively sub-modules it contains. Can someone
illustrate how this is done, e.g.

SRC_URI = "git://github.com/fluendo/gst-plugins-bad.git;protocol=git;branch=sdk-0.10.23-hls"
SRC_URI +="gitsm://?????"

This repo contains a "common" folder which is a Git sub-module.

Thanks . . .