fetch: Also fetch submodules in subdirectories in on-demand mode

When on-demand mode was active examining the new commits just fetched in
the superproject (to check if they record commits for submodules which are
not downloaded yet) wasn't done recursively. Because of that fetch did not
recursively fetch submodules living in subdirectories even when it should
have.

Fix that by adding the RECURSIVE flag to the diff_options used to check
the new commits and avoid future regressions in this area by moving a
submodule in t5526 into a subdirectory.

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