submodule: fix handling of superproject origin URLs like foo, ./foo and ./foo/bar

submodule: fix handling of superproject origin URLs like foo, ./foo and ./foo/bar

Currently git submodule init and git submodule sync fail with an error
if the superproject origin URL is of the form foo but succeed if the
superproject origin URL is of the form ./foo or ./foo/bar or foo/bar.

This change makes handling of the foo case behave like the handling
of the ./foo case and also ensures that superfluous leading and
embedded ./'s are removed from the resulting derived URLs.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>