manifestv2: implement slow readdelta() without revdiff

For manifest v2, revlog.revdiff() usually does not provide enough
information to produce a manifest. As a simple workaround, implement
readdelta() by reading both the old and the new manifest and use
manifest.diff() to find the difference. This is several times slower
than the current readdelta() for v1 manifests, but there seems to be
no other simple option, and this is still much faster than returning
the full manifest (at least for verify).

mercurial/manifest.py

@@ -613,7 +613,21 class manifest(revlog.revlog):
             return treemanifest('', data)
         return manifestdict(data)
 
+    def _slowreaddelta(self, node):
+        r0 = self.deltaparent(self.rev(node))
+        m0 = self.read(self.node(r0))
+        m1 = self.read(node)
+        md = self._newmanifest()
+        for f, ((n0, fl0), (n1, fl1)) in m0.diff(m1).iteritems():
+            if n1:
+                md[f] = n1
+                if fl1:
+                    md.setflag(f, fl1)
+        return md
+
     def readdelta(self, node):
+        if self._usemanifestv2:
+            return self._slowreaddelta(node)
         r = self.rev(node)
         d = mdiff.patchtext(self.revdiff(self.deltaparent(r), r))
         return self._newmanifest(d)