Add failing test to demonstrate wrong checkout behaviour

tests-clar/checkout/tree.c

@@ -85,3 +85,23 @@ void test_checkout_tree__calls_progress_callback(void)
 
 	cl_assert_equal_i(was_called, true);
 }
+
+void test_checkout_tree__doesnt_write_unrequested_files_to_worktree(void)
+{
+  git_oid master_oid;
+  git_oid chomped_oid;
+  git_commit* p_master_commit;
+  git_commit* p_chomped_commit;
+  git_checkout_opts opts = GIT_CHECKOUT_OPTS_INIT;
+
+  git_oid_fromstr(&master_oid, "a65fedf39aefe402d3bb6e24df4d4f5fe4547750");
+  git_oid_fromstr(&chomped_oid, "e90810b8df3e80c413d903f631643c716887138d");
+  cl_git_pass(git_commit_lookup(&p_master_commit, g_repo, &master_oid));
+  cl_git_pass(git_commit_lookup(&p_chomped_commit, g_repo, &chomped_oid));
+
+  /* A GIT_CHECKOUT_DEFAULT checkout is not allowed to add any file to the
+   * working tree from the index as it is supposed to be a dry run. */
+  opts.checkout_strategy = GIT_CHECKOUT_DEFAULT;
+  git_checkout_tree(g_repo, (git_object*)p_chomped_commit, &opts);
+  cl_assert_equal_i(false, git_path_isfile("testrepo/readme.txt"));
+}