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

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

Use the strvec_pushf() call that already appends a slash to also produce
the stuck form of the option --super-prefix instead of adding the option
name in a separate call of strvec_push() or strvec_pushl().  This way we
can more easily see that these parts make up a single option with its
argument and save a function call.

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