Fix tag name.

releaser.conf

@@ -2,6 +2,7 @@ name: "dulwich"
 news_file: "NEWS"
 pypi: "dulwich"
 timeout_days: 5
+tag_name: "dulwich-$VERSION"
 update_version {
   path: "setup.py"
   match: "^dulwich_version_string = '(.*)'$"