scm.create_patch no longer works for W3C test exporter

https://bugs.webkit.org/show_bug.cgi?id=245661
rdar://problem/100401276

Reviewed by Jonathan Bedard.

Exporter used to provide a folder for which getting the changes.
This does not seem to work anymore.
For that reason, do the filtering directly in test_exporter.py.

* Tools/Scripts/webkitpy/w3c/test_exporter.py:
(WebPlatformTestExporter._find_filename):
(WebPlatformTestExporter._is_ignored_file):

Canonical link: https://commits.webkit.org/254902@main

Tools/Scripts/webkitpy/w3c/test_exporter.py

@@ -157,10 +157,12 @@ def has_wpt_changes(self):
         return bool(self._wpt_patch)
 
     def _find_filename(self, line):
-        return line.split(b' ')[-1]
+        return line.split(b' ')[-1][2:]
 
     def _is_ignored_file(self, filename):
         filename = string_utils.decode(filename, target_type=str)
+        if not filename.startswith(WEBKIT_WPT_DIR):
+            return True
         for suffix in EXCLUDED_FILE_SUFFIXES:
             if filename.endswith(suffix):
                 return True