changelog: lazy decode description (API)

Currently, changelog reading decodes read values. This is wasteful
because a lot of times consumers aren't interested in some of these
values.

This patch changes description decoding to occur in changectx as
needed.

revsets reading changelog entries appear to speed up slightly:

revset  #7: author(lmoscovicz)
   plain
0) 0.906329
1) 0.872653

revset  #8: author(mpm)
   plain
0) 0.903478
1) 0.878037

revset  #9: author(lmoscovicz) or author(mpm)
   plain
0) 1.817855
1) 1.778680

revset  #10: author(mpm) or author(lmoscovicz)
   plain
0) 1.837052
1) 1.764568

mercurial/changelog.py

@@ -334,12 +334,20 b' class changelog(revlog.revlog):'
         (.*)            : comment (free text, ideally utf-8)
 
         changelog v0 doesn't use extra
+
+        Returns a 6-tuple consisting of the following:
+          - manifest node (binary)
+          - user (encoding.localstr)
+          - (time, timezone) 2-tuple of a float and int offset
+          - list of files modified by the cset
+          - commit message / description (binary)
+          - dict of extra entries
         """
         text = self.revision(node)
         if not text:
             return nullid, "", (0, 0), [], "", _defaultextra
         last = text.index("\n\n")
-        desc = encoding.tolocal(text[last + 2:])
+        desc = text[last + 2:]
         l = text[:last].split('\n')
         manifest = bin(l[0])
         user = encoding.tolocal(l[1])

mercurial/context.py

@@ -554,7 +554,7 b' class changectx(basectx):'
     def files(self):
         return self._changeset[3]
     def description(self):
-        return self._changeset[4]
+        return encoding.tolocal(self._changeset[4])
     def branch(self):
         return encoding.tolocal(self._changeset[5].get("branch"))
     def closesbranch(self):