context: use changelogrevision

Upcoming patches will make the changelogrevision object perform
lazy parsing. Let's switch to it.

Because we're switching from a tuple to an object, everthing that
accesses the internal cached attribute needs to be updated to access
via attributes. A nice side-effect is this makes the code easier to
read!

Surprisingly, this appears to make revsets accessing this data
slightly faster (values are before series, p1, this patch):

author(mpm)
0.896565
0.929984
0.914234

desc(bug)
0.887169
0.935642
0.921073

date(2015)
0.878797
0.908094
0.891980

extra(rebase_source)
0.865446
0.922624
0.912514

author(mpm) or author(greg)
1.801832
1.902112
1.860402

author(mpm) or desc(bug)
1.812438
1.860977
1.844850

date(2015) or branch(default)
0.968276
1.005824
0.994673

author(mpm) or desc(bug) or date(2015) or extra(rebase_source)
3.656193
3.743381
3.721032

mercurial/context.py

@@ -259,7 +259,7 b' class basectx(object):'
             if path in self._manifestdelta:
                 return (self._manifestdelta[path],
                         self._manifestdelta.flags(path))
-        node, flag = self._repo.manifest.find(self._changeset[0], path)
+        node, flag = self._repo.manifest.find(self._changeset.manifest, path)
         if not node:
             raise error.ManifestLookupError(self._node, path,
                                             _('not found in manifest'))
@@ -524,15 +524,15 b' class changectx(basectx):'
 
     @propertycache
     def _changeset(self):
-        return self._repo.changelog.read(self.rev())
+        return self._repo.changelog.changelogrevision(self.rev())
 
     @propertycache
     def _manifest(self):
-        return self._repo.manifest.read(self._changeset[0])
+        return self._repo.manifest.read(self._changeset.manifest)
 
     @propertycache
     def _manifestdelta(self):
-        return self._repo.manifest.readdelta(self._changeset[0])
+        return self._repo.manifest.readdelta(self._changeset.manifest)
 
     @propertycache
     def _parents(self):
@@ -543,24 +543,32 b' class changectx(basectx):'
         return [changectx(repo, p1), changectx(repo, p2)]
 
     def changeset(self):
-        return self._changeset
+        c = self._changeset
+        return (
+            c.manifest,
+            c.user,
+            c.date,
+            c.files,
+            c.description,
+            c.extra,
+        )
     def manifestnode(self):
-        return self._changeset[0]
+        return self._changeset.manifest
 
     def user(self):
-        return self._changeset[1]
+        return self._changeset.user
     def date(self):
-        return self._changeset[2]
+        return self._changeset.date
     def files(self):
-        return self._changeset[3]
+        return self._changeset.files
     def description(self):
-        return self._changeset[4]
+        return self._changeset.description
     def branch(self):
-        return encoding.tolocal(self._changeset[5].get("branch"))
+        return encoding.tolocal(self._changeset.extra.get("branch"))
     def closesbranch(self):
-        return 'close' in self._changeset[5]
+        return 'close' in self._changeset.extra
     def extra(self):
-        return self._changeset[5]
+        return self._changeset.extra
     def tags(self):
         return self._repo.nodetags(self._node)
     def bookmarks(self):