Teach push to respect the --dry-run option when configured to
recursively push submodules 'on-demand'.  This is done by passing the
--dry-run flag to the child process which performs a push for a
submodules when performing a dry-run.

In order to preserve good user experience, the additional check for
unpushed submodules is skipped during a dry-run when
--recurse-submodules=on-demand.  The check is skipped because the submodule
pushes were performed as dry-runs and this check would always fail as the
submodules would still need to be pushed.
Signed-off-by: NBrandon Williams <bmwill@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>