read-cache API & users: make discard_index() return void

The discard_index() function has not returned non-zero since
7a51ed66f6 (Make on-disk index representation separate from in-core
one, 2008-01-14), but we've had various code in-tree still acting as
though that might be the case.

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

builtin/merge.c

@@ -390,7 +390,8 @@ static void restore_state(const struct object_id *head,
 	run_command(&cmd);
 
 refresh_cache:
-	if (discard_cache() < 0 || read_cache() < 0)
+	discard_cache();
+	if (read_cache() < 0)
 		die(_("could not read index"));
 }