add a test to be sure unmatched branches raise an exception

tests/test_distgit.py

@@ -12,6 +12,13 @@ class TestDistGitNothing(unittest.TestCase):
         with self.assertRaises(TypeError):
             d = DistGitDirectory()
 
+class TestDistGitInvalid(unittest.TestCase):
+    def test_invalid_branchstring_raises(self):
+        self.branchstring = 'nope-not-a-branch'
+
+        with self.assertRaises(ValueError):
+            self.d = DistGitDirectory(self.branchstring)
+
 class TestDistgitOnlySig(unittest.TestCase):
     def setUp(self):
         self.branchstring = 'sig-cloud7'