revset: added lazyset implementation to desc revset

Performance benchmarking:

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

real  0m2.210s
user  0m2.158s
sys 0m0.049s

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

real  0m0.171s
user  0m0.131s
sys 0m0.035s

mercurial/revset.py

@@ -590,12 +590,12 b' def desc(repo, subset, x):'
     """
     # i18n: "desc" is a keyword
     ds = encoding.lower(getstring(x, _("desc requires a string")))
-    l = []
-    for r in subset:
-        c = repo[r]
-        if ds in encoding.lower(c.description()):
-            l.append(r)
-    return baseset(l)
+
+    def matches(x):
+        c = repo[x]
+        return ds in encoding.lower(c.description())
+
+    return lazyset(subset, matches)
 
 def _descendants(repo, subset, x, followfirst=False):
     args = getset(repo, baseset(repo), x)