discard revindex data when pack list changes

This is needed to fix verify-pack -v with multiple pack arguments.

Also, in theory, revindex data (if any) must be discarded whenever
reprepare_packed_git() is called. In practice this is hard to trigger
though.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -990,6 +990,7 @@ void prepare_packed_git(void)
 
 void reprepare_packed_git(void)
 {
+	discard_revindex();
 	prepare_packed_git_run_once = 0;
 	prepare_packed_git();
 }