changelog: lazily parse manifest node

Like the description, we store the raw bytes and convert from
hex on access.

This patch also marks the beginning of our new parsing method,
which is based on newline offsets and doesn't rely on
str.split().

Many revsets showed a performance improvement:

author(mpm)
0.896565
0.869085
0.868598

desc(bug)
0.887169
0.928164
0.910400

extra(rebase_source)
0.865446
0.871500
0.841644

author(mpm) or author(greg)
1.801832
1.791589
1.731503

author(mpm) or desc(bug)
1.812438
1.851003
1.798764

date(2015) or branch(default)
0.968276
0.974027
0.945792

mercurial/changelog.py

@@ -155,7 +155,7 b' class changelogrevision(object):'
         '_rawdesc',
         'extra',
         'files',
-        'manifest',
+        '_rawmanifest',
         'user',
     )
 
@@ -188,8 +188,10 b' class changelogrevision(object):'
         doublenl = text.index('\n\n')
         self._rawdesc = text[doublenl + 2:]
 
+        nl1 = text.index('\n')
+        self._rawmanifest = text[0:nl1]
+
         l = text[:doublenl].split('\n')
-        self.manifest = bin(l[0])
         self.user = encoding.tolocal(l[1])
 
         tdata = l[2].split(' ', 2)
@@ -211,6 +213,10 b' class changelogrevision(object):'
         return self
 
     @property
+    def manifest(self):
+        return bin(self._rawmanifest)
+
+    @property
     def description(self):
         return encoding.tolocal(self._rawdesc)