repo: ensure we can create repo at filesystem root

Test to ensure that we can create a repository at the filesystem
root.  Introduces a new test environment variable,
`GITTEST_INVASIVE_FILESYSTEM` for tests that do terrible things like
escaping the clar sandbox and writing to the root directory.  It is
expected that the CI builds will enable this but that normal people
would not want this.

tests/repo/init.c

@@ -714,3 +714,29 @@ void test_repo_init__init_with_initial_commit(void)
 
 	git_index_free(index);
 }
+
+void test_repo_init__at_filesystem_root(void)
+{
+	git_repository *repo;
+	const char *sandbox = clar_sandbox_path();
+	git_buf root = GIT_BUF_INIT;
+	int root_len;
+
+	if (!cl_getenv("GITTEST_INVASIVE_FILESYSTEM"))
+		cl_skip();
+
+	root_len = git_path_root(sandbox);
+	cl_assert(root_len >= 0);
+
+	git_buf_put(&root, sandbox, root_len+1);
+	git_buf_joinpath(&root, root.ptr, "libgit2_test_dir");
+
+	cl_assert(!git_path_exists(root.ptr));
+
+	cl_git_pass(git_repository_init(&repo, root.ptr, 0));
+	cl_assert(git_path_isdir(root.ptr));
+	cl_git_pass(git_futils_rmdir_r(root.ptr, NULL, GIT_RMDIR_REMOVE_FILES));
+
+	git_buf_free(&root);
+	git_repository_free(repo);
+}