Set Emacs config variable sentence-end-double-space.

Users using a non-English environment may have set this to `nil´, which
leads to fill-paragraph removing the second space.

* .dir-locals.el: Set sentence-end-double-space to true.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

.dir-locals.el

@@ -3,6 +3,7 @@
 ((nil
   . ((fill-column . 78)
      (tab-width   .  8)
+     (sentence-end-double-space . t)
 
      ;; For use with 'bug-reference-prog-mode'.
      (bug-reference-url-format . "http://bugs.gnu.org/%s")