In the updated delta editor path driver, ensure paths are relpaths.
This also fixes a bug with '/'-prefixed paths in svn_delta_path_driver2(). A path given as the string "/" was handled as a regular path (first open the editor root, then call the callback passing it the root parent-dir-baton) rather than as a root path.
* subversion/include/svn_delta.h (svn_delta_path_driver_cb_func2_t, svn_delta_path_driver3, svn_delta_path_driver_step): Declare that paths are relpaths. (svn_delta_path_driver2): Document that a slash prefix is allowed.
* subversion/libsvn_delta/deprecated.c (path_driver_2_to_3_baton_t, path_driver_2_to_3_func, svn_delta_path_driver2): Implement backward compatibility for a slash prefix.
* subversion/libsvn_delta/path_driver.c (svn_delta_path_driver3): Rename variables to 'relpath', for clarity. (svn_delta_path_driver_step): Assert the path is a relpath, remove support for a slash prefix, and rename variables.