t/helper: add 'find-pack' test-tool

In a following commit, we will make it possible to separate objects in
different packfiles depending on a filter.

To make sure that the right objects are in the right packs, let's add a
new test-tool that can display which packfile(s) a given object is in.

Let's also make it possible to check if a given object is in the
expected number of packfiles with a `--check-count <n>` option.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Makefile b/Makefile
index 003e63b..f267034 100644
--- a/Makefile
+++ b/Makefile
@@ -800,6 +800,7 @@
 TEST_BUILTINS_OBJS += test-env-helper.o
 TEST_BUILTINS_OBJS += test-example-decorate.o
 TEST_BUILTINS_OBJS += test-fast-rebase.o
+TEST_BUILTINS_OBJS += test-find-pack.o
 TEST_BUILTINS_OBJS += test-fsmonitor-client.o
 TEST_BUILTINS_OBJS += test-genrandom.o
 TEST_BUILTINS_OBJS += test-genzeros.o