* Modified populate_db_from_tree() to make a deep copy of argument.
* Wrote and debugged build_tree_data_from_db().
* Wrote and debugged strip_fields().
* Debugged diff_trees().
* Finished test_dir_target() in test_delroot.
* Fixed comment for test_delroot.test_dir_target().
* Added and debugged test_delroot.test_file_target().