Add stagit lister

That lister is really near the cgit & gitweb implementations. But the dom data is again
structured differently though so this implementation stands on its own.

Refs. swh/meta#5048

swh/lister/tests/test_cli.py

@@ -48,6 +48,9 @@ lister_args = {
     "gitiles": {
         "instance": "gerrit.googlesource.com",
     },
+    "stagit": {
+        "url": "https://git.codemadness.org",
+    },
 }