submodule sync: Update "submodule.<name>.url" for empty directories
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: Andreas Köhler <andi5.py@gmx.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>