obsutil: make sure "addedrevs" is not None in getobsoleted()

If it were None, "rev in addedrevs" would crash. So tr.changes["revs"]
should be a mandatory parameter here.

mercurial/obsutil.py

@@ -465,7 +465,7 b' def getobsoleted(repo, tr):'
     succsmarkers = repo.obsstore.successors.get
     public = phases.public
     addedmarkers = tr.changes.get('obsmarkers')
-    addedrevs = tr.changes.get('revs')
+    addedrevs = tr.changes['revs']
     seenrevs = set()
     obsoleted = set()
     for mark in addedmarkers: