Add debian upstream metadata.

debian/upstream/metadata

@@ -0,0 +1,6 @@
+Name: dulwich
+Repository: https://www.dulwich.io/code/
+Repository-Browse: https://www.dulwich.io/code/
+Bug-Database: https://github.com/dulwich/dulwich/issues
+Bug-Submit: https://github.com/dulwich/dulwich/issues/new
+Contact: https://groups.google.com/forum/#!forum/dulwich-discuss