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>

builtin/receive-pack.c

@@ -277,10 +277,12 @@ static int show_one_alternate_sha1(const unsigned char sha1[20], void *unused)
 	return 0;
 }
 
-static void collect_one_alternate_ref(const struct ref *ref, void *data)
+static void collect_one_alternate_ref(const char *refname,
+				      const struct object_id *oid,
+				      void *data)
 {
 	struct sha1_array *sa = data;
-	sha1_array_append(sa, ref->old_oid.hash);
+	sha1_array_append(sa, oid->hash);
 }
 
 static void write_head_info(void)