Rename the refspec_item_init() function introduced in
6d4c057859 ("refspec: introduce struct refspec", 2018-05-16) to
refspec_item_init_or_die().
This follows the convention of other *_or_die() functions, and is done
in preparation for making it a wrapper for a non-fatal variant.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>