feat(_comp_compgen_commands): include dirs

It makes sense to offer completions from explicitly specified paths,
too.

Thanks-to: Koichi Murase <myoga.murase@gmail.com>

bash_completion

@@ -2708,7 +2708,7 @@ _complete_as_root()
 _comp_compgen_commands()
 {
     [[ ! ${cur-} ]] && shopt -q no_empty_cmd_completion && return
-    _comp_compgen -- -c
+    _comp_compgen -- -c -o plusdirs
     # for e.g. spaces in paths to and in command names
     ((${#COMPREPLY[@]} == 0)) || compopt -o filenames
 }