Add git-index-pack stress test

.gitignore

+tmp/

Makefile

+test:	tmp/linux.pack tmp/test-repo.git
+	cat tmp/linux.pack | (cd tmp/test-repo.git && time git index-pack --stdin)
+
+tmp/linux.pack:	tmp
+	curl https://s3.amazonaws.com/gitlab-internal-file-hosting/test-data/pack-78486715cddada143192c51c8e82ea14369b91df.pack > $@
+
+tmp/test-repo.git:
+	git init --bare $@
+
+tmp:
+	mkdir tmp

README.md

@@ -3,5 +3,5 @@
 Usage:
 ```
-time ./index-pack-test
+make test
 ```