fetch/pull: Add the 'on-demand' value to the --recurse-submodules option

fetch/pull: Add the 'on-demand' value to the --recurse-submodules option

Until now the --recurse-submodules option could only be used to either
fetch all populated submodules recursively or to disable recursion
completely. As fetch and pull now by default just fetch those submodules
for which new commits have been fetched in the superproject, a command
line option to enforce that behavior is needed to be able to override
configuration settings.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>