check_packed_git_idx(): check integrity of the idx file itself.

Although pack-check.c had routine to verify the checksum for the
pack index file itself, the core did not check it before using
it.

This is stolen from the patch to tighten packname requirements.

Signed-off-by: Junio C Hamano <junkio@cox.net>
(cherry picked from 797bd6f490 commit)

t/t5300-pack-object.sh

@@ -163,6 +163,13 @@ test_expect_success \
      else :;
      fi &&
 
+     cp test-1-${packname_1}.pack test-3.pack &&
+     dd if=/dev/zero of=test-3.idx count=1 bs=1 conv=notrunc seek=1200 &&
+     if git-verify-pack test-3.pack
+     then false
+     else :;
+     fi &&
+
      :'
 
 test_expect_success \