revset: added lazyset implementation to merge revset

Performance benchmarking:

$ time hg log -qr "first(merge())"
102:58039eddbdda

real  0m0.276s
user  0m0.208s
sys 0m0.047s

$ time ./hg log -qr "first(merge())"
102:58039eddbdda

real  0m0.192s
user  0m0.154s
sys 0m0.027s

mercurial/revset.py

@@ -992,7 +992,7 b' def merge(repo, subset, x):'
     # i18n: "merge" is a keyword
     getargs(x, 0, 0, _("merge takes no arguments"))
     cl = repo.changelog
-    return baseset([r for r in subset if cl.parentrevs(r)[1] != -1])
+    return lazyset(subset, lambda r: cl.parentrevs(r)[1] != -1)
 
 def branchpoint(repo, subset, x):
     """``branchpoint()``