relnotes: enable extension when running releasenotes command

Last night, when I was working on different machine, the test worked, now on a
different machine it does not. Looks like we should make sure releasenotes
extension is loaded.

Differential Revision: https://phab.mercurial-scm.org/D4425

contrib/relnotes

@@ -110,6 +110,8 b' def main():'
     fromext = subprocess.check_output(
         [
             "hg",
+            "--config",
+            "extensions.releasenotes=",
             "releasenotes",
             "-r",
             "%s::%s" % (args.startrev[0], args.stoprev[0]),