While trying to make test pass for #318, I've noticed that jruby tests are really flaky.

See https://travis-ci.org/schacon/ruby-git/builds/182204463, https://travis-ci.org/schacon/ruby-git/builds/182015600 and https://travis-ci.org/schacon/ruby-git/builds/182011837 (virtually same code).

This makes contribution hard to tests.