Generate links when diary & wiki dir are the same

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.

autoload/vimwiki/path.vim

@@ -20,12 +20,11 @@ else
   endfunction
 endif
 
-
-" collapse sections like /a/b/../c to /a/c
+" collapse sections like /a/b/../c to /a/c and /a/b/./c to /a/b/c
 function! vimwiki#path#normalize(path) abort
   let path = a:path
   while 1
-    let result = substitute(path, '/[^/]\+/\.\.', '', '')
+    let result = substitute(path, '/[^/]\+/\.\.\|\./', '', '')
     if result ==# path
       break
     endif