Revert "mkrockspecs: slingshot doesn't need slingshot submodule git magic!" and others...

This reverts commits 039367682f360cec97324e3fdb58bc3443938468,
d963b9832cd68fb938bc50ff423b9c6231e00abc and most of
0613d6e152f698bc4f570f00aed14103e8ed6683.

The whole issue was a red-herring, better solved by a tiny one-line
fix to bootstrap $GIT detection.