[blinkpy] Recurse into subtrees when checking diff

`git diff-tree` should be used with the `-r` flag if we want to recurse
into subtrees (e.g. to check if css/css-animations is affected by a
commit as opposed to just css/). This is the only place where we forgot
to do that and it has caused some false positives of "affecting this
directory" in our bug filing.

Fixed: 850088
Change-Id: Ifdd1b9eeba2c5749effdab1041d86c6500160ce7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590014
Commit-Queue: Robert Ma <robertma@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Auto-Submit: Robert Ma <robertma@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836670}

third_party/blink/tools/blinkpy/w3c/local_wpt.py

@@ -202,8 +202,8 @@ class LocalWPT(object):
     def is_commit_affecting_directory(self, commit, directory):
         """Checks if a commit affects a directory."""
         exit_code = self.run([
-            'git', 'diff-tree', '--quiet', '--no-commit-id', commit, '--',
-            directory
+            'git', 'diff-tree', '--quiet', '--no-commit-id', '-r', commit,
+            '--', directory
         ],
                              return_exit_code=True)
         return exit_code == 1

third_party/blink/tools/blinkpy/w3c/local_wpt_unittest.py

@@ -146,8 +146,8 @@ class LocalWPTTest(unittest.TestCase):
         self.assertTrue(
             local_wpt.is_commit_affecting_directory('HEAD', 'css/'))
         self.assertEqual(host.executive.calls, [[
-            'git', 'diff-tree', '--quiet', '--no-commit-id', 'HEAD', '--',
-            'css/'
+            'git', 'diff-tree', '--quiet', '--no-commit-id', '-r', 'HEAD',
+            '--', 'css/'
         ]])
 
     def test_seek_change_id(self):