add logic to handle Unicode beta period file names

	In downloader.rb, add logic to handle file names of the form
	UnicodeData-12.0.0d6.txt. To find the right file, we download
	the index of the directory. Then we download the files by finding
	the file names from the index. Files are always checked for changes,
	because changes might be frequent during the beta period.
	We also check whether any index.html files are left when we are not
	in the beta period. This would indicate that we might have stale
	data from the beta period rather than the actual release data.

	simplify filename-related code

	(Thanks to Nobuyoshi Nakada for the hint!)

	downloader.rb: keep linked file newer than cached file

	* tool/downloader.rb (Downloader.save_cache): keep linked file
	  newer than cached file, so that GNU make triggers when the
	  content is updated.  it uses the timestamp of symlink itself
	  instead of the target.

	add puts statements to debug Unicode file download (temporary)

	Unicode file download doesn't work with Visual Studio, see e.g.
	https://ci.appveyor.com/project/ruby/ruby/builds/23614399/job/f8vya2l7fjdfcye4
	We temporarily produce more output for debugging.

	* remove trailing spaces.

	output more debug information in downloader.rb (temporary)

	Unicode file download doesn't work with Visual Studio, we need more debug output.

	downloader.rb: quote base name

	downloader.rb: fix typo, extra %

	revert r67445, (r67446,) r67447

	Debugging output is no longer needed because the problem has been fixed with r67449.

	appveyor.yml: Use pre-generated headers and tables

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67528 b2dd03c8-39d4-4d8f-98ff-823fe69b080e