Reject paths containing control characters at the repos layer.
Newly added paths are checked by the repos commit editor, and rejected if they contain control characters.
Related to issue #4340.
* subversion/libsvn_repos/commit.c (illegal_path_escape): New, copied from libsvn_subr/path.c. Going forward, we could make this function public in libsvn_subr. But a static copy in this file will make backports easier for now. (check_cntrl): New, based on svn_path_check_valid(). (add_file_or_directory): Reject paths which contain control characters.
* subversion/tests/libsvn_repos/repos-test.c (filename_with_control_chars): New test. (test_funcs): Add new test.