push: --dry-run updates submodules when --recurse-submodules=on-demand

push: --dry-run updates submodules when --recurse-submodules=on-demand

This patch adds a test to illustrate how push run with --dry-run doesn't
actually perform a dry-run when push is configured to push submodules
on-demand.  Instead all submodules which need to be pushed are actually
pushed to their remotes while any updates for the superproject are
performed as a dry-run.  This is a bug and not the intended behaviour of
a dry-run.

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