graphmod: stop building a list out of branchgroupiter

Not only does this defeat the purpose of having `branchgroupiter` an iterable, but
this also makes revs something else than a smartset preventing to use revs.min()
in the call to reachablerevs.

mercurial/graphmod.py

@@ -247,7 +247,8 b' def dagwalker(repo, revs):'
         if firstbranchrevset:
             firstbranch = repo.revs(firstbranchrevset)
         parentrevs = repo.changelog.parentrevs
-        revs = list(groupbranchiter(revs, parentrevs, firstbranch))
+        revs = groupbranchiter(revs, parentrevs, firstbranch)
+        revs = revset.baseset(revs)
 
     for rev in revs:
         ctx = repo[rev]