baseset: implement a fastasc and fastdesc

Baseset contains already-computed revisions. It is considered "cheap" to do
operations on an already-computed set. So we add attributes to hold version of
the list in ascending and descending order and use them for `fastasc` and
`fastdesc`. Having distinct lists is important to provide correct iteration in
all cases. Altering a python list will impact an iterator connected to it.

eg: not preserving order at iterator creation time

    >>> l = [0, 1]
    >>> i = iter(l)
    >>> l.reverse()
    >>> list(i)
    [1, 0]

eg: corrupting in progress iteration

    >>> l = [0, 1]
    >>> i = iter(l)
    >>> i.next()
    0
    >>> l.reverse()
    >>> i.next()
    0

mercurial/revset.py

@@ -2332,6 +2332,18 b' class baseset(abstractsmartset):'
         self._list = data
         self._set = None
 
+    @util.propertycache
+    def _asclist(self):
+        asclist = self._list[:]
+        asclist.sort()
+        return asclist
+
+    def fastasc(self):
+        return iter(self._asclist)
+
+    def fastdesc(self):
+        return reversed(self._asclist)
+
     def set(self):
         """Returns a set or a smartset containing all the elements.