[PATCH 3 of 3] revset: improve roots revset performance

[Durham Goode]

# HG changeset patch
# User Durham Goode <durham at fb.com>
# Date 1396307014 25200
#      Mon Mar 31 16:03:34 2014 -0700
# Node ID 157d96db8eaf11d343017477847ba8719d52eeca
# Parent  aa93084956fd536092f56eb2347dcb723e153d94
revset: improve roots revset performance

Previously we would iterate over every item in the subset, checking if it was in
the provided args. This often meant iterating over every rev in the repo.

Now we iterate over the args provided, checking if they exist in the subset.
On a large repo this brings setting phase boundaries (which use this revset
roots(X:: - X::Y)) down from 0.8 seconds to 0.4 seconds.

The "roots((tip~100::) - (tip~100::tip))" revset in revsetbenchmarks shows it
going from 0.12s to 0.10s, so we should be able to catch regressions here in the
future.

This actually introduces a regression in 'roots(all())' (0.2s to 0.26s) since
we're now using spansets, which are slightly slower to do containment checks on.
I believe this trade off is worth it, since it makes the revset O(number of
args) instead of O(size of repo).

diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -1480,8 +1480,8 @@
     """``roots(set)``
     Changesets in set with no parent changeset in set.
     """
-    s = getset(repo, baseset(repo.changelog), x).set()
-    subset = baseset([r for r in subset if r in s])
+    s = getset(repo, spanset(repo), x).set()
+    subset = baseset([r for r in s if r in subset.set()])
     cs = _children(repo, subset, s)
     return subset - cs