Improve backward compatibility for svn_delta_path_driver2().
This ensures the input array is not modified, and changes the behaviour to be less surprising.
* subversion/include/svn_delta.h, subversion/libsvn_delta/deprecated.c (svn_delta_path_driver2): Add back slash prefixes if any, rather than the first, of the inputs had one. Duplicate the array if modifying it.
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.