revset: improve _descendants performance

Previously revset._descendants would iterate over the entire subset (which is
often the entire repo) and test if each rev was in the descendants list. This is
really slow on large repos (3+ seconds).

Now we iterate over the descendants and test if they're in the subset.
This affects advancing and retracting the phase boundary (3.5 seconds down to
0.8 seconds, which is even faster than it was in 2.9). Also affects commands
that move the phase boundary (commit and rebase, presumably).

The new revsetbenchmark indicates an improvement from 0.2 to 0.12 seconds. So
future revset changes should be able to notice regressions.

I removed a bad test. It was recently added and tested '1:: and reverse(all())',
which has an amibiguous output direction.  Previously it printed in reverse order,
because we iterated over the subset (the reverse part). Now it prints in normal
order because we iterate over the 1:: . Since the revset itself doesn't imply an
order, I removed the test.

contrib/revsetbenchmarks.txt

@@ -12,4 +12,5 @@ max(tip:0)
 min(0:tip)
 0::
 min(0::)
+roots((tip~100::) - (tip~100::tip))
 ::p1(p1(tip))::

mercurial/revset.py

@@ -661,8 +661,18 @@ def _descendants(repo, subset, x, followfirst=False):
     if not args:
         return baseset([])
     s = _revdescendants(repo, args, followfirst)
-    a = set(args)
-    return subset.filter(lambda r: r in s or r in a)
+
+    # Both sets need to be ascending in order to lazily return the union
+    # in the correct order.
+    args.ascending()
+
+    subsetset = subset.set()
+    result = (orderedlazyset(s, subsetset.__contains__, ascending=True) +
+              orderedlazyset(args, subsetset.__contains__, ascending=True))
+
+    # Wrap result in a lazyset since it's an _addset, which doesn't implement
+    # all the necessary functions to be consumed by callers.
+    return orderedlazyset(result, lambda r: True, ascending=True)
 
 def descendants(repo, subset, x):
     """``descendants(set)``

tests/test-revset.t

@@ -474,16 +474,6 @@ min: empty on unordered set
   2
   1
   0
-  $ log '1:: and reverse(all())'
-  9
-  8
-  7
-  6
-  5
-  4
-  3
-  2
-  1
   $ log 'rev(5)'
   5
   $ log 'sort(limit(reverse(all()), 3))'