From @perlun

I think the root cause here is that our code is not really thread safe, and that's why JRuby (which is much more concurrent than the evil-GIL-threaded MRI) triggers these errors.
[..]
The error:

Error: test_git_init_bare(TestThreadSafety): Git::GitExecuteError: git '--git-dir=/tmp/d20180328-6779-88i76v/.git' init '--bare' 2>&1:fatal: GIT_WORK_TREE (or --work-tree=) not allowed without specifying GIT_DIR (or --git-dir=)
...indicates that something goes wrong; it does specify the --git-dir but git still complains about this.

I read this SO thread but I couldn't see the GIT_WORK_TREE variable being set on the Travis machines.