Force t5302 to use a single thread

If the packs are made using multiple threads, they are no longer identical
on the 4-core Xeon I tested on.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5302-pack-index.sh

@@ -10,6 +10,7 @@ test_expect_success \
     'setup' \
     'rm -rf .git
      git init &&
+     git config pack.threads 1 &&
      i=1 &&
      while test $i -le 100
      do