[PATCH 3 of 3 diffset-extra] revset: use _diffset in spanset.__sub__

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1410200439 25200
#      Mon Sep 08 11:20:39 2014 -0700
# Node ID b12f25dad733d8842472bfb64f8fa80347cad2f0
# Parent  3a75a1f6937b15a6edc5e7a5c838c8d4cdf9dc73
revset: use _diffset in spanset.__sub__

This patch results in the following statistically significant changes in
revset benchmarks for a clone of the Firefox repository.

revset #8: author(mpm)
wall 11.409557 comb 11.410000 user 11.360000 sys 0.050000 (best of 3)
wall 12.045913 comb 12.040000 user 12.000000 sys 0.040000 (best of 3)

revset #9: author(lmoscovicz) or author(mpm)
wall 30.106085 comb 30.100000 user 29.980000 sys 0.120000 (best of 3)
wall 36.646229 comb 36.640000 user 36.490000 sys 0.150000 (best of 3)

revset #10: author(mpm) or author(lmoscovicz)
wall 29.797047 comb 29.800000 user 29.690000 sys 0.110000 (best of 3)
wall 35.162670 comb 35.170000 user 35.010000 sys 0.160000 (best of 3)

revset #22: max(::(tip~20) - obsolete())
wall 0.966111 comb 0.970000 user 0.960000 sys 0.010000 (best of 10)
wall 1.167777 comb 1.170000 user 1.160000 sys 0.010000 (best of 8)

revset #24: (not public() - obsolete())
wall 0.365095 comb 0.360000 user 0.360000 sys 0.000000 (best of 25)
wall 0.219435 comb 0.220000 user 0.220000 sys 0.000000 (best of 42)

Since all but one of these is a regression, there are merits for
rejecting this patch.

diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -2873,14 +2873,14 @@ class spanset(_orderedsetmixin):
         else:
             return orderedlazyset(self, x.__contains__, ascending=False)
 
     def __sub__(self, x):
-        if isinstance(x, baseset):
-            x = x.set()
-        if self._start <= self._end:
-            return orderedlazyset(self, lambda r: r not in x)
-        else:
-            return orderedlazyset(self, lambda r: r not in x, ascending=False)
+        kwargs = {}
+        if self.isascending() and x.isascending():
+            kwargs['ascending'] = True
+        if self.isdescending() and x.isdescending():
+            kwargs['ascending'] = False
+        return _diffset(self, x, **kwargs)
 
     def __add__(self, x):
         kwargs = {}
         if self.isascending() and x.isascending():