submodule: use strvec_pushf() for --super-prefix

submodule: use strvec_pushf() for --super-prefix

absorb_git_dir_into_superproject() uses a strbuf and strvec_pushl() to
build and add the --super-prefix option and its argument.  Use a single
strvec_pushf() call to add the stuck form instead, which reduces the
code size and avoids a strbuf allocation and release.  The same is
already done in submodule_reset_index() and submodule_move_head().

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>