Fix extra escaping on prompt filename compl on Win

This also unifies code of add_filename_completion() method for two
platforms.
Correct add_filename_completion() styling

* Add leading comment.
* Use `[]` instead of ` * `.
Merge branch 'fix-win-prompt-file-name-escaping'

Fix extra escaping on prompt filename completion on Windows.

This was done properly for non-Windows platforms, but was wrong for
Windows.