revset: extract function that creates range set from computed revisions

So we can pass m=0 to _makerangeset() even if the revision 0 is hidden.
Hidden revisions are filtered by spanset.

mercurial/revset.py

@@ -366,8 +366,9 def rangeset(repo, subset, x, y, order):
 
     if not m or not n:
         return baseset()
-    m, n = m.first(), n.last()
-
+    return _makerangeset(repo, subset, m.first(), n.last(), order)
+
+def _makerangeset(repo, subset, m, n, order):
     if m == n:
         r = baseset([m])
     elif n == node.wdirrev: