push: propagate remote and refspec with --recurse-submodules

push: propagate remote and refspec with --recurse-submodules

Teach "push --recurse-submodules" to propagate, if given a name as remote, the
provided remote and refspec recursively to the pushes performed in the
submodules. The push will therefore only succeed if all submodules have a
remote with such a name configured.

Note that "push --recurse-submodules" with a path or URL as remote will not
propagate the remote or refspec and instead use the default remote and refspec
configured in the submodule, preserving the current behavior.

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