Log Message:
devel/git-cinnabar: Move GITHUB_SUBMODULES to Makefile.common.

This makes no difference for devel/git-cinnabar, and unbreaks
devel/py-hg-cinnabarclone by making the new patch under git-core/
apply.  This patch isn't needed for py-hg-cinnabarclone but it'll be
easier to maintain, I think, if we have all the git-cinnabar in one
place.