treemanifest: add configuration for using treemanifest type

This change adds boolean configuration option
experimental.treemanifest. When the option is enabled, manifests are
parsed into the new treemanifest type.

Tests can be now run using treemanifest by switching the config option
default in localrepo._applyrequirements(). Tests pass even when made
to randomly choose between manifestdict and treemanifest, suggesting
that the two types produce identical manifests (so e.g. a manifest
revlog entry written from a treemanifest can be parsed by the
manifestdict code).

mercurial/localrepo.py

@@ -331,6 +331,9 b' class localrepository(object):'
         manifestcachesize = self.ui.configint('format', 'manifestcachesize')
         if manifestcachesize is not None:
             self.svfs.options['manifestcachesize'] = manifestcachesize
+        usetreemanifest = self.ui.configbool('experimental', 'treemanifest')
+        if usetreemanifest is not None:
+            self.svfs.options['usetreemanifest'] = usetreemanifest
 
     def _writerequirements(self):
         reqfile = self.vfs("requires", "w")

mercurial/manifest.py

@@ -556,16 +556,24 b' class manifest(revlog.revlog):'
         # revs at a time (such as during commit --amend). When rebasing large
         # stacks of commits, the number can go up, hence the config knob below.
         cachesize = 4
+        usetreemanifest = False
         opts = getattr(opener, 'options', None)
         if opts is not None:
             cachesize = opts.get('manifestcachesize', cachesize)
+            usetreemanifest = opts.get('usetreemanifest', usetreemanifest)
         self._mancache = util.lrucachedict(cachesize)
         revlog.revlog.__init__(self, opener, "00manifest.i")
+        self._usetreemanifest = usetreemanifest
+
+    def _newmanifest(self, data=''):
+        if self._usetreemanifest:
+            return treemanifest(data)
+        return manifestdict(data)
 
     def readdelta(self, node):
         r = self.rev(node)
         d = mdiff.patchtext(self.revdiff(self.deltaparent(r), r))
-        return manifestdict(d)
+        return self._newmanifest(d)
 
     def readfast(self, node):
         '''use the faster of readdelta or read'''
@@ -577,12 +585,12 b' class manifest(revlog.revlog):'
 
     def read(self, node):
         if node == revlog.nullid:
-            return manifestdict() # don't upset local cache
+            return self._newmanifest() # don't upset local cache
         if node in self._mancache:
             return self._mancache[node][0]
         text = self.revision(node)
         arraytext = array.array('c', text)
-        m = manifestdict(text)
+        m = self._newmanifest(text)
         self._mancache[node] = (m, arraytext)
         return m
 
@@ -596,7 +604,7 b' class manifest(revlog.revlog):'
             return None, None
 
     def add(self, m, transaction, link, p1, p2, added, removed):
-        if p1 in self._mancache:
+        if p1 in self._mancache and not self._usetreemanifest:
             # If our first parent is in the manifest cache, we can
             # compute a delta here using properties we know about the
             # manifest up-front, which may save time later for the