Build: Init submodule for Travis demo building

Use the already cloned wet-boew-dist to save the extra full clone. Thereare notes on using the `--reference` command on https://www.kernel.org/pub/software/scm/git/docs/git-clone.html, but those shouldn't be affected by this usage.
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This builds on the work in #2511, but I found an issue with the difference between the raw Git clone vs the GH4W in the treatment of submodules.