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>

builtin/index-pack.c

@@ -884,6 +884,8 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage(index_pack_usage);
 
+	read_replace_refs = 0;
+
 	/*
 	 * We wish to read the repository's config file if any, and
 	 * for that it is necessary to call setup_git_directory_gently().

t/t6050-replace.sh

@@ -219,6 +219,12 @@ test_expect_success 'bisect and replacements' '
      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