parse-options API: add a usage_msg_optf()

parse-options API: add a usage_msg_optf()

Add a usage_msg_optf() as a shorthand for the sort of
usage_msg_opt(xstrfmt(...)) used in builtin/stash.c. I'll make more
use of this function in builtin/cat-file.c shortly.

The disconnect between the "..." and "fmt" is a bit unusual, but it
works just fine and this keeps it consistent with usage_msg_opt(),
i.e. a caller of it can be moved to usage_msg_optf() and not have to
have its arguments re-arranged.

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