revset: added lazyset implementation to author revset

Performance benchmarking:

$ time hg log -qr "first(author(mpm))"
0:9117c6561b0b

real  0m3.486s
user  0m3.317s
sys 0m0.077s

$ time ./hg log -qr "first(author(mpm))"
0:9117c6561b0b

real  0m0.551s
user  0m0.295s
sys 0m0.072s

mercurial/revset.py

@@ -347,8 +347,7 b' def author(repo, subset, x):'
     # i18n: "author" is a keyword
     n = encoding.lower(getstring(x, _("author requires a string")))
     kind, pattern, matcher = _substringmatcher(n)
-    return baseset([r for r in subset if
-        matcher(encoding.lower(repo[r].user()))])
+    return lazyset(subset, lambda x: matcher(encoding.lower(repo[x].user())))
 
 def bisect(repo, subset, x):
     """``bisect(string)``