If a submodule directory has not been filled by "git submodule update"
yet, then "git submodule sync" must still update the super-project's
configuration for submodule.<name>.url.

This situation occurs when switching between branches with a module from
different urls and other branches without the submodule.
Signed-off-by: NAndreas Köhler <andi5.py@gmx.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>