Udpate git url.

debian/upstream/metadata

@@ -1,4 +1,4 @@
-Repository: https://www.dulwich.io/code/dulwich/
-Repository-Browse: https://www.dulwich.io/code/dulwich/
+Repository: https://github.com/dulwich/dulwich.git
+Repository-Browse: https://github.com/dulwich/dulwich
 Bug-Database: https://github.com/dulwich/dulwich/issues
 Bug-Submit: https://github.com/dulwich/dulwich/issues/new