smartset: add first and last methods

smartset: add first and last methods

In multiple places in the code, we use `someset[0]` or `someset[-1]`. This
works only because the `someset` is usually a baseset. For the same reason we
introduce a `first` and `last` methods to be implemented for all smartset
classes.

--- a/mercurial/revset.py	Tue Oct 07 00:20:00 2014 -0700
+++ b/mercurial/revset.py	Mon Oct 06 11:46:53 2014 -0700
@@ -2262,6 +2262,18 @@
             raise ValueError('arg is an empty sequence')
         return max(self)
 
+    def first(self):
+        """return the first element in the set (user iteration perspective)
+
+        Return None if the set is empty"""
+        raise NotImplementedError()
+
+    def last(self):
+        """return the last element in the set (user iteration perspective)
+
+        Return None if the set is empty"""
+        raise NotImplementedError()
+
     def reverse(self):
         """reverse the expected iteration order"""
         raise NotImplementedError()