revset-last: remove user of baseset.append

A `baseset` has multiple cached results and will get even more in the future.
Making it an object "populated once" like the other smartsets makes it both safer
and simpler. The append method will be removed at some point.

mercurial/revset.py

@@ -1043,16 +1043,16 b' def last(repo, subset, x):'
     ss = subset.set()
     os = getset(repo, spanset(repo), l[0])
     os.reverse()
-    bs = baseset()
+    result = []
     it = iter(os)
     for x in xrange(lim):
         try:
             y = it.next()
             if y in ss:
-                bs.append(y)
+                result.append(y)
         except (StopIteration):
             break
-    return bs
+    return baseset(result)
 
 def maxrev(repo, subset, x):
     """``max(set)``