Don't take into account commit message for skip-if filter

If there is some skip-if condition containing all-files-match-any, then
Zuul skips jobs for changes without modified files (merge commits),
because it always matches '/COMMIT_MSG'.

So test files for regexes only if CR has more than one modified file,
because '/COMMIT_MSG' is always included even for empty merge commits.

Change-Id: Iad78d9eb8212beea3238728321c1ba74efa991e2
diff --git a/tests/test_model.py b/tests/test_model.py
index 2711618..739eef3 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -31,12 +31,12 @@
 
     def test_change_matches_returns_false_for_matched_skip_if(self):
         change = model.Change('project')
-        change.files = ['docs/foo']
+        change.files = ['/COMMIT_MSG', 'docs/foo']
         self.assertFalse(self.job.changeMatches(change))
 
     def test_change_matches_returns_true_for_unmatched_skip_if(self):
         change = model.Change('project')
-        change.files = ['foo']
+        change.files = ['/COMMIT_MSG', 'foo']
         self.assertTrue(self.job.changeMatches(change))
 
     def test_copy_retains_skip_if(self):