Refactoring: show_dir and show_files

This de-duplicates the shared code, and adds some (first) improvements
to not stat all of v:oldfiles (twice).

If accepted, it allows for easily adding a cache for abs_path handling
etc.

 - Add _show_filtered_oldfiles and _get_filtered_oldfiles
 - Fixes g:startify_files_number<1
Refactoring: l:cnt -> s:entry_number
Refactoring: make "dir" list always relative
Refactoring: fix last line usage