obsolete: explicitly pass relation items to effectflag computation

To track folds, we are about to change the content of `rel`. To simplify this
change, we update the `geteffectflag` function beforehand.

mercurial/obsolete.py

@@ -1016,7 +1016,7 b' def createmarkers(repo, relations, flag='
             if saveeffectflag:
                 # The effect flag is saved in a versioned field name for future
                 # evolution
-                effectflag = obsutil.geteffectflag(rel)
+                effectflag = obsutil.geteffectflag(prec, sucs)
                 localmetadata[obsutil.EFFECTFLAGFIELD] = "%d" % effectflag
 
             # Creating the marker causes the hidden cache to become invalid,

mercurial/obsutil.py

@@ -413,15 +413,13 b' def _cmpdiff(leftctx, rightctx):'
             return False
     return True
 
-def geteffectflag(relation):
+def geteffectflag(source, successors):
     """ From an obs-marker relation, compute what changed between the
     predecessor and the successor.
     """
     effects = 0
 
-    source = relation[0]
-
-    for changectx in relation[1]:
+    for changectx in successors:
         # Check if description has changed
         if changectx.description() != source.description():
             effects |= DESCCHANGED