Set fill column to 70 in .dir-locals.el.

This controls where comments and other text wraps.  70 is the default
value, so this simply returns it to the default for people who have
overridden it.  Most notmuch code already adheres to this.

.dir-locals.el

@@ -20,4 +20,6 @@
   (tab-width . 8)
   (sh-basic-offset . 4)
   (sh-indentation . 4))
+ (nil
+  (fill-column . 70))
  )