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

doc/source/zuul.rst

@@ -759,7 +759,10 @@ each job as it builds a list from the project specification.
     expressions.
 
     The pattern for '/COMMIT_MSG' is always matched on and does not
-    have to be included.
+    have to be included. Exception is merge commits (without modified
+    files), in this case '/COMMIT_MSG' is not matched, and job is not
+    skipped. In case of merge commits it's assumed that list of modified
+    files isn't predictible and CI should be run.
 
 **voting (optional)**
   Boolean value (``true`` or ``false``) that indicates whatever

tests/test_change_matcher.py

@@ -123,13 +123,13 @@ class TestMatchAllFiles(BaseTestMatcher):
         self._test_matches(False)
 
     def test_matches_returns_false_when_not_all_files_match(self):
-        self._test_matches(False, files=['docs/foo', 'foo/bar'])
+        self._test_matches(False, files=['/COMMIT_MSG', 'docs/foo', 'foo/bar'])
 
-    def test_matches_returns_true_when_commit_message_matches(self):
-        self._test_matches(True, files=['/COMMIT_MSG'])
+    def test_matches_returns_false_when_commit_message_matches(self):
+        self._test_matches(False, files=['/COMMIT_MSG'])
 
     def test_matches_returns_true_when_all_files_match(self):
-        self._test_matches(True, files=['docs/foo'])
+        self._test_matches(True, files=['/COMMIT_MSG', 'docs/foo'])
 
 
 class TestMatchAll(BaseTestMatcher):

tests/test_model.py

@@ -31,12 +31,12 @@ class TestJob(BaseTestCase):
 
     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):

zuul/change_matcher.py

@@ -101,7 +101,7 @@ class MatchAllFiles(AbstractMatcherCollection):
         yield self.commit_regex
 
     def matches(self, change):
-        if not (hasattr(change, 'files') and change.files):
+        if not (hasattr(change, 'files') and len(change.files) > 1):
             return False
         for file_ in change.files:
             matched_file = False