sidedatacopies: get and store sidedata in the changelogrevision object

The object provide a simple way to access changelog entry, we need it to also
bear the sidedata value.

Since the sidedata are retrieved at the same time as the revision, we can do
that without extra cost.

Differential Revision: https://phab.mercurial-scm.org/D6951

mercurial/changelog.py

@@ -268,9 +268,10 b' class changelogrevision(object):'
     __slots__ = (
         r'_offsets',
         r'_text',
+        r'_sidedata',
     )
 
-    def __new__(cls, text):
+    def __new__(cls, text, sidedata):
         if not text:
             return _changelogrevision(extra=_defaultextra)
 
@@ -302,6 +303,7 b' class changelogrevision(object):'
 
         self._offsets = (nl1, nl2, nl3, doublenl)
         self._text = text
+        self._sidedata = sidedata
 
         return self
 
@@ -613,12 +615,13 b' class changelog(revlog.revlog):'
         ``changelogrevision`` instead, as it is faster for partial object
         access.
         """
-        c = changelogrevision(self.revision(node))
+        c = changelogrevision(*self._revisiondata(node))
         return (c.manifest, c.user, c.date, c.files, c.description, c.extra)
 
     def changelogrevision(self, nodeorrev):
         """Obtain a ``changelogrevision`` for a node or revision."""
-        return changelogrevision(self.revision(nodeorrev))
+        text, sidedata = self._revisiondata(nodeorrev)
+        return changelogrevision(text, sidedata)
 
     def readfiles(self, node):
         """