changelog: lazily parse description

Before, the description field was converted to a localstr at parse
time. With this patch, we store the raw description and convert to
a localstr when it is first accessed.

We see a revset speedup for revsets that don't access the description:

author(mpm)
0.896565
0.914234
0.869085

date(2015)
0.878797
0.891980
0.862525

extra(rebase_source)
0.865446
0.912514
0.871500

author(mpm) or author(greg)
1.801832
1.860402
1.791589

date(2015) or branch(default)
0.968276
0.994673
0.974027

author(mpm) or desc(bug) or date(2015) or extra(rebase_source)
3.656193
3.721032
3.643593

As you can see, most of these revsets are already faster than from
before this refactoring: we have already offset the performance
loss from the introduction of the new class representing parsed
changelog entries!

mercurial/changelog.py

@@ -152,7 +152,7 b' class changelogrevision(object):'
 
     __slots__ = (
         'date',
-        'description',
+        '_rawdesc',
         'extra',
         'files',
         'manifest',
@@ -185,9 +185,10 b' class changelogrevision(object):'
         #
         # changelog v0 doesn't use extra
 
-        last = text.index("\n\n")
-        self.description = encoding.tolocal(text[last + 2:])
-        l = text[:last].split('\n')
+        doublenl = text.index('\n\n')
+        self._rawdesc = text[doublenl + 2:]
+
+        l = text[:doublenl].split('\n')
         self.manifest = bin(l[0])
         self.user = encoding.tolocal(l[1])
 
@@ -209,6 +210,10 b' class changelogrevision(object):'
 
         return self
 
+    @property
+    def description(self):
+        return encoding.tolocal(self._rawdesc)
+
 class changelog(revlog.revlog):
     def __init__(self, opener):
         revlog.revlog.__init__(self, opener, "00changelog.i")