Make sure GIT_COMMITTER{_NAME,_EMAIL} is set.

dulwich/tests/compat/test_porcelain.py

@@ -90,7 +90,11 @@ class TagCreateSignTestCase(PorcelainGpgTestCase, CompatTestCase):
                 "foo",
                 "verifyme",
             ],
-            env={'GNUPGHOME': os.environ['GNUPGHOME']},
+            env={
+                'GNUPGHOME': os.environ['GNUPGHOME'],
+                'GIT_COMMITTER_NAME': 'Joe Example',
+                'GIT_COMMITTER_EMAIL': 'joe@example.com',
+                },
         )
         tag = self.repo[b"refs/tags/verifyme"]
         self.assertNotEqual(tag.signature, None)