[ruby/fileutils] Make mkdir_p only attempt to create necessary directories

Previously, if creating the directory directly didn't work
and the directory didn't exist, mkdir_p would create all
directories from the root.  This modifies the approach to
check whether the directory exists when walking up the
directory tree from the argument, and once you have found an
intermediate directory that exists, you only need to create
directories under it.

This approach has a couple advantages:

1) It performs better when most directories in path already exist,
and that will be true for most usage of mkdir_p, as mkdir_p is
usually called with paths where the first few directories exist
and only the last directory or last few directories do not.

2) It works in file-system access limited environments such as
when unveil(2) is used on OpenBSD.  In these environments, if
/foo/bar/baz exists and is unveiled, you can do
`mkdir /foo/bar/baz/xyz` but `mkdir /foo` and `mkdir /foo/bar` raise
Errno::ENOENT.

https://github.com/ruby/fileutils/commit/ec0c229e78

lib/fileutils.rb

@@ -223,8 +223,9 @@ module FileUtils
       until path == stack.last   # dirname("/")=="/", dirname("C:/")=="C:/"
         stack.push path
         path = File.dirname(path)
+        break if File.directory?(path)
       end
-      stack.pop                 # root directory should exist
+      stack.pop if path == stack.last   # root directory should exist
       stack.reverse_each do |dir|
         begin
           fu_mkdir dir, mode