manifestv2: add (unused) config option

manifestv2: add (unused) config option

With tree manifests, hashes will change anyway, so now is a good time
to also take up the old plans of a new manifest format. While there
should be little or no reason to use tree manifests with the current
manifest format (v1) once the new format (v2) is supported, we'll try
to keep the two dimensions (flat/tree and v1/v2) separate.

In preparation for adding a the new format, let's add configuration
for it and propagate that configuration to the manifest revlog
subclass. The new configuration ("experimental.manifestv2") says in
what format to write the manifest data. We may later add other
configuration to choose how to hash it, either keeping the v1 hash for
BC or hashing the v2 content.

See http://mercurial.selenic.com/wiki/ManifestV2Plan for more details.

--- a/mercurial/localrepo.py	Fri Mar 27 15:37:46 2015 -0700
+++ b/mercurial/localrepo.py	Fri Mar 27 16:19:44 2015 -0700
@@ -334,6 +334,9 @@
         usetreemanifest = self.ui.configbool('experimental', 'treemanifest')
         if usetreemanifest is not None:
             self.svfs.options['usetreemanifest'] = usetreemanifest
+        usemanifestv2 = self.ui.configbool('experimental', 'manifestv2')
+        if usemanifestv2 is not None:
+            self.svfs.options['usemanifestv2'] = usemanifestv2
 
     def _writerequirements(self):
         reqfile = self.vfs("requires", "w")

--- a/mercurial/manifest.py	Fri Mar 27 15:37:46 2015 -0700
+++ b/mercurial/manifest.py	Fri Mar 27 16:19:44 2015 -0700
@@ -597,13 +597,16 @@
         # stacks of commits, the number can go up, hence the config knob below.
         cachesize = 4
         usetreemanifest = False
+        usemanifestv2 = False
         opts = getattr(opener, 'options', None)
         if opts is not None:
             cachesize = opts.get('manifestcachesize', cachesize)
             usetreemanifest = opts.get('usetreemanifest', usetreemanifest)
+            usemanifestv2 = opts.get('usemanifestv2', usemanifestv2)
         self._mancache = util.lrucachedict(cachesize)
         revlog.revlog.__init__(self, opener, "00manifest.i")
         self._usetreemanifest = usetreemanifest
+        self._usemanifestv2 = usemanifestv2
 
     def _newmanifest(self, data=''):
         if self._usetreemanifest: