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: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>