tests/pack: do strict check of testpack's SHA1 hash

tests-clar/pack/packbuilder.c

 #include "clar_libgit2.h"
+#include "fileops.h"
+#include "hash.h"
 #include "iterator.h"
 #include "vector.h"
 #include "posix.h"
@@ -76,6 +78,10 @@ static void seed_packbuilder(void)
 void test_pack_packbuilder__create_pack(void)
 {
 	git_transfer_progress stats;
+	git_buf buf = GIT_BUF_INIT;
+	git_hash_ctx ctx;
+	git_oid hash;
+	char hex[41]; hex[40] = '\0';
 
 	seed_packbuilder();
 	cl_git_pass(git_packbuilder_write(_packbuilder, "testpack.pack"));
@@ -83,6 +89,32 @@ void test_pack_packbuilder__create_pack(void)
 	cl_git_pass(git_indexer_new(&_indexer, "testpack.pack"));
 	cl_git_pass(git_indexer_run(_indexer, &stats));
 	cl_git_pass(git_indexer_write(_indexer));
+
+	/*
+	 * By default, packfiles are created with only one thread.
+	 * Therefore we can predict the object ordering and make sure
+	 * we create exactly the same pack as git.git does when *not*
+	 * reusing existing deltas (as libgit2).
+	 *
+	 * $ cd tests-clar/resources/testrepo.git
+	 * $ git rev-list --objects HEAD | \
+	 * 	git pack-objects -q --no-reuse-delta --threads=1 pack
+	 * $ sha1sum git-80e61eb315239ef3c53033e37fee43b744d57122.pack
+	 * 5d410bdf97cf896f9007681b92868471d636954b
+	 *
+	 */
+
+	cl_git_pass(git_futils_readbuffer(&buf, "testpack.pack"));
+
+	cl_git_pass(git_hash_init(&ctx));
+	cl_git_pass(git_hash_update(&ctx, buf.ptr, buf.size));
+	cl_git_pass(git_hash_final(&hash, &ctx));
+
+	git_buf_free(&buf);
+
+	git_oid_fmt(hex, &hash);
+
+	cl_assert_equal_s(hex, "5d410bdf97cf896f9007681b92868471d636954b");
 }
 
 static git_transfer_progress stats;