revset: added lazyset implementation to keyword revset

Performance benchmarking:

$ time hg log -qr "first(keyword(changeset))"
0:9117c6561b0b

real  0m3.466s
user  0m3.345s
sys 0m0.072s

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

real  0m0.365s
user  0m0.199s
sys 0m0.083s

mercurial/revset.py

@@ -921,13 +921,13 b' def keyword(repo, subset, x):'
     """
     # i18n: "keyword" is a keyword
     kw = encoding.lower(getstring(x, _("keyword requires a string")))
-    l = []
-    for r in subset:
+
+    def matches(r):
         c = repo[r]
-        if util.any(kw in encoding.lower(t)
-                    for t in c.files() + [c.user(), c.description()]):
-            l.append(r)
-    return baseset(l)
+        return util.any(kw in encoding.lower(t) for t in c.files() + [c.user(),
+            c.description()])
+
+    return lazyset(subset, matches)
 
 def limit(repo, subset, x):
     """``limit(set, [n])``