Change the "Missing OWNERS" presubmit check to only suggest owners for missing files.

tests/presubmit_unittest.py

Index: tests/presubmit_unittest.py
diff --git a/tests/presubmit_unittest.py b/tests/presubmit_unittest.py
index 34f0b489e90e2489ccc6416ccbc29177be5cb4a3..6bdc76fe1d5b063acc6ee8f31bd1e66feacbaa21 100755
--- a/tests/presubmit_unittest.py
+++ b/tests/presubmit_unittest.py
@@ -2494,7 +2494,7 @@ class CannedChecksUnittest(PresubmitTestsBase):
         fake_db.files_not_covered_by(set(['foo/xyz.cc']),
             people).AndReturn(uncovered_files)
       if not is_committing and uncovered_files:
-        fake_db.reviewers_for(set(['foo/xyz.cc']),
+        fake_db.reviewers_for(set(['foo']),
             change.author_email).AndReturn(change.author_email)
 
     self.mox.ReplayAll()