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

Similar reasoning as in 52e6171ec822 (obsutil: make sure "addedrevs"
is not None in getobsoleted(), 2018-08-25).

Differential Revision: https://phab.mercurial-scm.org/D4411

mercurial/obsutil.py

@@ -464,7 +464,7 b' def getobsoleted(repo, tr):'
     phase = repo._phasecache.phase
     succsmarkers = repo.obsstore.successors.get
     public = phases.public
-    addedmarkers = tr.changes.get('obsmarkers')
+    addedmarkers = tr.changes['obsmarkers']
     origrepolen = tr.changes['origrepolen']
     seenrevs = set()
     obsoleted = set()