wpt-import: Fix a failure on "git add"

This CL fixes a regression since r467828. It changed the working directory, but
it missed to update relative path in "git add" argument.

BUG=710535
TBR=jeffcarp@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2848183002
Cr-Commit-Position: refs/heads/master@{#468264}

third_party/WebKit/Tools/Scripts/webkitpy/w3c/test_importer.py

@@ -230,7 +230,7 @@ class TestImporter(object):
         test_copier = TestCopier(self.host, temp_repo_path)
         test_copier.do_import()
 
-        self.run(['git', 'add', '--all', 'LayoutTests/external/%s' % dest_dir_name])
+        self.run(['git', 'add', '--all', 'external/%s' % dest_dir_name])
 
         self._delete_orphaned_baselines(dest_path)