refspec: add back a refspec_item_init() function

refspec: add back a refspec_item_init() function

Re-add the non-fatal version of refspec_item_init_or_die() renamed
away in an earlier change to get a more minimal diff. This should be
used by callers that have their own error handling.

This new function could be marked "static" since nothing outside of
refspec.c uses it, but expecting future use of it, let's make it
available to other users.

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