unpack-trees: oneway_merge to update submodules

unpack-trees: oneway_merge to update submodules

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>