This is due to the use of withPermDir (followed by a rename), which 
creates the temporary directory with these permissions for security 
reasons. We should use our umask to (recursively) set the permissions 
before renaming the temporary directory back to the actual one.

The following patch sent by Ben Franksen <ben.franksen@online.de> updated issue issue2725 with
status=resolved;resolvedin=2.18.1

Hash: 3979f2a1a11d913aeb8bbecb4c821db4ec75a6c0
Author: Ben Franksen <ben.franksen@online.de>
* resolve issue2725: permissions of _darcs/patch_index

  The cheap solution here is to set the permissions of the newly created
  _darcs/patch_index/ to that of its parent directory _darcs/. Using _darcs/
  has the advantage that darcs now automatically fixes the permissions of
  existing patch_index directories.