index-pack: Don't follow replace refs.

Without this, attempting to index a pack containing objects that have been
replaced results in a fatal error that looks like:

fatal: SHA1 COLLISION FOUND WITH <replaced-object> !

Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
Acked-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t6050-replace.sh b/t/t6050-replace.sh
index 203ffdb..4185b7c 100755
--- a/t/t6050-replace.sh
+++ b/t/t6050-replace.sh
@@ -219,6 +219,12 @@
      git bisect reset
 '
 
+test_expect_success 'index-pack and replacements' '
+	git --no-replace-objects rev-list --objects HEAD |
+	git --no-replace-objects pack-objects test- &&
+	git index-pack test-*.pack
+'
+
 #
 #
 test_done