GitHub file matching support

Allow to configure jobs to run only when certain files are changed.

Github does not list the /COMMIT_MSG in the changed files as gerrit
does. Therefore the matcher now returns False only if the single file is
the /COMMIT_MSG one.

Change-Id: I4fa8a328f2ba430c25377e50e1eff7c45829eba6
diff --git a/tests/unit/test_change_matcher.py b/tests/unit/test_change_matcher.py
index 0585322..6b161a1 100644
--- a/tests/unit/test_change_matcher.py
+++ b/tests/unit/test_change_matcher.py
@@ -125,12 +125,18 @@
     def test_matches_returns_false_when_not_all_files_match(self):
         self._test_matches(False, files=['/COMMIT_MSG', 'docs/foo', 'foo/bar'])
 
+    def test_matches_returns_true_when_single_file_does_not_match(self):
+        self._test_matches(True, files=['docs/foo'])
+
     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=['/COMMIT_MSG', 'docs/foo'])
 
+    def test_matches_returns_true_when_single_file_matches(self):
+        self._test_matches(True, files=['docs/foo'])
+
 
 class TestMatchAll(BaseTestMatcher):