receive-pack, fetch-pack: reject bogus pack that records objects twice

When receive-pack & fetch-pack are run and store the pack obtained over
the wire to a local repository, they internally run the index-pack command
with the --strict option. Make sure that we reject incoming packfile that
records objects twice to avoid spreading such a damage.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 0945adb..98025da 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1122,8 +1122,10 @@
 		if (!index_name)
 			die("--verify with no packfile name given");
 		read_idx_option(&opts, index_name);
-		opts.flags |= WRITE_IDX_VERIFY;
+		opts.flags |= WRITE_IDX_VERIFY | WRITE_IDX_STRICT;
 	}
+	if (strict)
+		opts.flags |= WRITE_IDX_STRICT;
 
 	curr_pack = open_pack_file(pack_name);
 	parse_pack_header();