obsolete: simplify relevantmarker

Drop duplicate assignment from a merge failure. Save
one loop iteration by exploiting that pendingnodes will
be seennodes after the first round anyway, so just
pre-initialize the set accordingly. From Anton Shestakov's
review on !867. Performance difference for my test case is
in the noise.

mercurial/obsolete.py

@@ -799,8 +799,7 b' class obsstore:'
                     if rev is not None and rev in revs:
                         pendingnodes.add(node)
         seenmarkers = set()
-        seenmarkers = set()
-        seennodes = set()
+        seennodes = set(pendingnodes)
         precursorsmarkers = self.predecessors
         succsmarkers = self.successors
         children = self.children