Restore directory completions for subcommands

Fixes #6798

This re-adds some false positives: functions, builtins and abbreviations
are suggested after commands like sudo but I don't think anyone had
complained about that.

share/functions/__fish_complete_external_command.fish

@@ -1,3 +1,3 @@
 function __fish_complete_external_command
-    command find $PATH/ -maxdepth 1 -perm -u+x 2>&- | string match -r '[^/]*$'
+    complete -C "$argv[1]"
 end

share/functions/__fish_complete_subcommand.fish

@@ -49,7 +49,7 @@ function __fish_complete_subcommand -d "Complete subcommand" --no-scope-shadowin
     end
 
     if test $allow_functions_and_builtins = false && test (count $subcommand) -eq 1
-        __fish_complete_external_command
+        __fish_complete_external_command "$subcommand"
     else
         printf "%s\n" (complete -C "$subcommand")
     end