for_each_alternate_ref: pass name/oid instead of ref struct
Breaking down the fields in the interface makes it easier to
change the backend of for_each_alternate_ref to something
that doesn't use "struct ref" internally.
The only field that callers actually look at is the oid,
anyway. The refname is kept in the interface as a plausible
thing for future code to want.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>