changeset 20713:6a1a4c212d50

revset: improve head revset performance

Previously the head() revset would iterate over every item in the subset and
check if it was a head.  Since the subset is often the entire repo, this was
slow on large repos. Now we iterate over each item in the head list and check if
it's in the subset, which results in much less work.

hg log -r 'head()' on a large repo:
Before: 0.95s
After: 0.28s

--- a/mercurial/revset.py	Tue Mar 11 16:52:15 2014 -0700
+++ b/mercurial/revset.py	Thu Mar 13 13:47:21 2014 -0700
@@ -940,7 +940,7 @@
     hs = set()
     for b, ls in repo.branchmap().iteritems():
         hs.update(repo[h].rev() for h in ls)
-    return subset.filter(lambda r: r in hs)
+    return baseset(hs).filter(subset.__contains__)
 
 def heads(repo, subset, x):
     """``heads(set)``