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>

pack-revindex.h

@@ -7,5 +7,6 @@ struct revindex_entry {
 };
 
 struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs);
+void discard_revindex(void);
 
 #endif