When generating links, we first check that the file is not a diary file
as we don't want to include those in the list. That work is delegated to
the `is_diary_file` function. Prior to this change, the function always
returned true if the file was in the diary directory.
This approach gives false positives for a wiki which has a flat structure
and the wiki files and diary files share a directory. eg:
let wiki.diary_rel_path = './'
This change reuses existing diary functions from the diary.vim module to
get an exact list of diary files to check against.
b96e82d6cc