treemanifest: drop 22nd byte for consistency with manifestdict

When assigning a 22-byte hash to a nodeid in a manifest, manifestdict
drops the 22nd byte, while treemanifest keeps it. Let's make
treemanifest drop the 22nd byte as well.

mercurial/manifest.py

@@ -447,7 +447,7 b' class treemanifest(object):'
                 self._dirs[dir] = treemanifest(self._subpath(dir))
             self._dirs[dir].__setitem__(subpath, n)
         else:
-            self._files[f] = n
+            self._files[f] = n[:21] # to match manifestdict's behavior
 
     def setflag(self, f, flags):
         """Set the flags (symlink, executable) for path f."""