[PATCH] better git-submodule status output

This patch makes the output of git-submodule more useful to handle the
management of a repository using multiple submodules. Namely, it
displays informations about how the current checked out version relates
to the registered one (i.e. direct parent, direct child, "just
different", registered revision does not exist), and displays if the
submodules contain uncommitted changes.

This (among other things) allows to do git-submodule update while
knowing exactly what will happen.

Sylvain