[PATCH 2 of 2 V2] revset: add only() fastpath for minus operations

[Durham Goode]

# HG changeset patch
# User Durham Goode <durham at fb.com>
# Date 1455823717 28800
#      Thu Feb 18 11:28:37 2016 -0800
# Node ID a2a62c7df76324d49046a2893c971195507ee64b
# Parent  dc8d72c3df7dfcd7b67fa71d679cc7c3ad90f25c
revset: add only() fastpath for minus operations

Previously, '::X - ::Y' was translated to '::X and not ::Y' and the 'and'
optimizer turned this into 'only(X, Y)'. Since we no longer transform minus into
and-not we lost this optimization.

This patch adds it back. This results in no performance change actually (as
determined by revsetbenchmark.py), but it does undo the test changes caused by
the use of the minus operator.

diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -2170,6 +2170,19 @@ def optimize(x, small):
         wa, ta = optimize(x[1], True)
         wb, tb = optimize(x[2], True)
 
+        # (::x - ::y) has a fast path
+        def isonly(revs, bases):
+            return (
+                revs is not None
+                and revs[0] == 'func'
+                and getstring(revs[1], _('not a symbol')) == 'ancestors'
+                and bases is not None
+                and bases[0] == 'func'
+                and getstring(bases[1], _('not a symbol')) == 'ancestors')
+
+        if isonly(ta, tb):
+            return wa, ('func', ('symbol', 'only'), ('list', ta[2], tb[2]))
+
         return wa, (op, ta, tb)
     elif op == 'only':
         return optimize(('func', ('symbol', 'only'),
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -1248,16 +1248,13 @@ check that conversion to only works
     (dagrangepre
       ('symbol', '1')))
   * optimized:
-  (minus
-    (func
-      ('symbol', 'ancestors')
-      ('symbol', '3'))
-    (func
-      ('symbol', 'ancestors')
+  (func
+    ('symbol', 'only')
+    (list
+      ('symbol', '3')
       ('symbol', '1')))
   * set:
-  <filteredset
-    <generatorset+>>
+  <baseset+ [3]>
   3
   $ try --optimize 'ancestors(1) - ancestors(3)'
   (minus
@@ -1268,16 +1265,13 @@ check that conversion to only works
       ('symbol', 'ancestors')
       ('symbol', '3')))
   * optimized:
-  (minus
-    (func
-      ('symbol', 'ancestors')
-      ('symbol', '1'))
-    (func
-      ('symbol', 'ancestors')
+  (func
+    ('symbol', 'only')
+    (list
+      ('symbol', '1')
       ('symbol', '3')))
   * set:
-  <filteredset
-    <generatorset+>>
+  <baseset+ []>
   $ try --optimize 'not ::2 and ::6'
   (and
     (not