When there is a one way merge, each submodule needs to be one way merged
as well, if we're asked to recurse into submodules.
In case of a submodule, check if it is up-to-date, otherwise set the
flag CE_UPDATE, which will trigger an update of it in the phase updating
the tree later.
Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>