cachevfs: add a vfs dedicated to cache

Most of the cache content lives in '.hg/cache/'. Moreover they are computed
exclusively from data in the '.hg/store' directory. This creates issues with
the share extension as the '.hg/store' directory is shared but the '.hg/cache'
is not. On large repositories, this makes this prevent some usage of the share
extension inefficient as some caches can take minutes to be recomputed.

To improve the situation, we introduce a new 'cachevfs' that will be dedicated
to cache reading and writing. In the next patches of this series, we'll
migrate the 4 existing caches to it and update the share extension.

mercurial/localrepo.py

@@ -423,6 +423,8 b' class localrepository(object):'
         self.svfs = self.store.vfs
         self.sjoin = self.store.join
         self.vfs.createmode = self.store.createmode
+        self.cachevfs = vfsmod.vfs(self.vfs.join('cache'))
+        self.cachevfs.createmode = self.store.createmode
         if (self.ui.configbool('devel', 'all-warnings') or
             self.ui.configbool('devel', 'check-locks')):
             if util.safehasattr(self.svfs, 'vfs'): # this is filtervfs

mercurial/statichttprepo.py

@@ -124,6 +124,7 b' class statichttprepository(localrepo.loc'
 
         vfsclass = build_opener(ui, authinfo)
         self.vfs = vfsclass(self.path)
+        self.cachevfs = vfsclass(self.vfs.join('cache'))
         self._phasedefaults = []
 
         self.names = namespaces.namespaces()