Fix watch file.

debian/watch

@@ -1,3 +1,3 @@
 version=3
 opts="pgpsigurlmangle=s/$/.asc/" \
-	https://pypi.python.org/packages/source/d/dulwich/dulwich-(.*).tar.gz
+	https://pypi.python.org/pypi/dulwich/ https://pypi.python.org/packages/source/d/dulwich/dulwich-(.*)\.tar\.gz