Use -- before seq for negative numbers

busybox seq was complaining about the command:

    seq -550 -1

because it was trying to interpret -550 as a flag. Use -- to prevent
this.

tests/checks/return.fish

@@ -26,7 +26,7 @@ function empty_return
     return $argv[1]
 end
 
-for i in (seq -550 -1)
+for i in (seq -- -550 -1)
     empty_return $i
     if test $status -eq 0
         echo returning $i from a fish script maps to a $status of 0!