shelve: compress on-disk bundle

Shelve is using 'HG10UN' bundle for no obvious reason. Changing this to 'HG10BZ'
as we do for other bundle stored on disk.

hgext/shelve.py

@@ -106,7 +106,7 b' class shelvedfile(object):'
         return bundlerepo.bundlerepository(self.repo.baseui, self.repo.root,
                                            self.vfs.join(self.fname))
     def writebundle(self, cg):
-        changegroup.writebundle(self.ui, cg, self.fname, 'HG10UN', self.vfs)
+        changegroup.writebundle(self.ui, cg, self.fname, 'HG10BZ', self.vfs)
 
 class shelvedstate(object):
     """Handle persistence during unshelving operations.