submodule--helper: use xstrfmt() in clone_submodule()

submodule--helper: use xstrfmt() in clone_submodule()

Use xstrfmt() in clone_submodule() instead of a "struct strbuf" in two
cases where we weren't getting anything out of using the "struct
strbuf".

This changes code that was was added along with other uses of "struct
strbuf" in this function in ee8838d1577 (submodule: rewrite
`module_clone` shell function in C, 2015-09-08).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>