Simplify loop to find out segments to be created

Doing it this way is simpler and it doesn't end up adding "/" to the
list of folders, so it doesn't need to be removed later.

lib/fileutils.rb

@@ -220,12 +220,10 @@ def mkdir_p(list, mode: nil, noop: nil, verbose: nil)
       end
 
       stack = []
-      until path == stack.last   # dirname("/")=="/", dirname("C:/")=="C:/"
+      until File.directory?(path)
         stack.push path
         path = File.dirname(path)
-        break if File.directory?(path)
       end
-      stack.pop if path == stack.last   # root directory should exist
       stack.reverse_each do |dir|
         begin
           fu_mkdir dir, mode