manifest: change manifestctx to not inherit from manifestdict

manifest: change manifestctx to not inherit from manifestdict

If manifestctx inherits from manifestdict, it requires some weird logic to
lazily load the dict if a piece of information is asked for. This ended up being
complicated and unintuitive to use.

Let's move the dict creation to .read(). This will make even more sense once we
start adding readdelta() and other similar methods to manifestctx.

--- a/mercurial/context.py	Mon Aug 29 18:02:09 2016 -0700
+++ b/mercurial/context.py	Wed Aug 31 12:46:53 2016 -0700
@@ -528,7 +528,7 @@
 
     @propertycache
     def _manifest(self):
-        return self._repo.manifestlog[self._changeset.manifest]
+        return self._repo.manifestlog[self._changeset.manifest].read()
 
     @propertycache
     def _manifestdelta(self):

--- a/mercurial/manifest.py	Mon Aug 29 18:02:09 2016 -0700
+++ b/mercurial/manifest.py	Wed Aug 31 12:46:53 2016 -0700
@@ -962,12 +962,13 @@
             self._mancache[node] = m
         return m
 
-class manifestctx(manifestdict):
+class manifestctx(object):
     """A class representing a single revision of a manifest, including its
     contents, its parent revs, and its linkrev.
     """
     def __init__(self, revlog, node):
         self._revlog = revlog
+        self._data = None
 
         self._node = node
 
@@ -978,21 +979,26 @@
         #rev = revlog.rev(node)
         #self.linkrev = revlog.linkrev(rev)
 
-        # This should eventually be made lazy loaded, so consumers can access
-        # the node/p1/linkrev data without having to parse the whole manifest.
-        data = revlog.revision(node)
-        arraytext = array.array('c', data)
-        revlog._fulltextcache[node] = arraytext
-        super(manifestctx, self).__init__(data)
-
     def node(self):
         return self._node
 
-class treemanifestctx(treemanifest):
+    def read(self):
+        if not self._data:
+            if self._node == revlog.nullid:
+                self._data = manifestdict()
+            else:
+                text = self._revlog.revision(self._node)
+                arraytext = array.array('c', text)
+                self._revlog._fulltextcache[self._node] = arraytext
+                self._data = manifestdict(text)
+        return self._data
+
+class treemanifestctx(object):
     def __init__(self, revlog, dir, node):
         revlog = revlog.dirlog(dir)
         self._revlog = revlog
         self._dir = dir
+        self._data = None
 
         self._node = node
 
@@ -1003,19 +1009,26 @@
         #rev = revlog.rev(node)
         #self.linkrev = revlog.linkrev(rev)
 
-        if revlog._treeondisk:
-            super(treemanifestctx, self).__init__(dir=dir)
-            def gettext():
-                return revlog.revision(node)
-            def readsubtree(dir, subm):
-                return revlog.dirlog(dir).read(subm)
-            self.read(gettext, readsubtree)
-            self.setnode(node)
-        else:
-            text = revlog.revision(node)
-            arraytext = array.array('c', text)
-            revlog.fulltextcache[node] = arraytext
-            super(treemanifestctx, self).__init__(dir=dir, text=text)
+    def read(self):
+        if not self._data:
+            if self._node == revlog.nullid:
+                self._data = treemanifest()
+            elif self._revlog._treeondisk:
+                m = treemanifest(dir=self._dir)
+                def gettext():
+                    return self._revlog.revision(self._node)
+                def readsubtree(dir, subm):
+                    return treemanifestctx(self._revlog, dir, subm).read()
+                m.read(gettext, readsubtree)
+                m.setnode(self._node)
+                self._data = m
+            else:
+                text = self._revlog.revision(self._node)
+                arraytext = array.array('c', text)
+                self._revlog.fulltextcache[self._node] = arraytext
+                self._data = treemanifest(dir=self._dir, text=text)
+
+        return self._data
 
     def node(self):
         return self._node