Fix ref calculation for ZUUL_CHANGES legacy var

We need the last 2 digits from the change number to define XX in
refs/changes/XX/512270/2. Currently, we get everything but those
last two digits.

Change-Id: I84ebe2d048574e07e3eb838379c10854d775f131

zuul/ansible/filter/zuul_filters.py

@@ -36,7 +36,7 @@ def zuul_legacy_vars(zuul):
             ['%s:%s:refs/changes/%s/%s/%s' % (
                 i['project']['name'],
                 i['branch'],
-                str(i['change'])[:-2:],
+                str(i['change'])[-2:],
                 i['change'],
                 i['patchset'])
              for i in zuul['items']])