submodule: no [--merge|--rebase] when newly cloned

submodule: no [--merge|--rebase] when newly cloned

"git submodule update" can be run with either the "--merge" or "--rebase"
option, or submodule.<name>.update configuration variable can be set to
"merge" or "rebase, to cause local work to get integrated when updating
the submodule.

When a submodule is newly cloned, however, it does not have a check out
when a rebase or merge is attempted, leading to a failure.  For newly
cloned submodules, simply check out the appropriate revision.  There is no
local work to integrate with for them.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Acked-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>