Since git 1.7.8 the $GIT_DIR of the submodule can be a reference to the
super-module's $GIT_DIR/modules/<name>/

Test for existence instead of testing for a directory before attempting
a git clone there.

Change-Id: Ic539e770067da1417a2ad374c21253212473abe6
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>