Fix relative paths for VimwikiRenameLink. Merge PR #787.

Fixes #90 

__Problem__: VimwikiRenameLink does not rename dir1/toto url in dir2/tata
because the dirs (dir2 and .) were well crossed but badly inspected.
__Solution__: `blob(.ext*)` -> `glob(**/*.ext)` + find the relative URL

* Fasten: VimwikiRenameLink goes faster with cache
More: To compute old_url regex, use a cache dict because it is the same
for files in the same directories

* Util DeleteHiddenBuffer -> do not delete Vader buffer

* Test: Prettify: Util: teardown delete defined function

* Remove unnecessary trailing spaces
Note: list_VimwikiReturn.vader has some necessary trailing spaces

autoload/vimwiki/path.vim

@@ -97,6 +97,10 @@ endfunction
 
 " Returns: the relative path from a:dir to a:file
 function! vimwiki#path#relpath(dir, file) abort
+  " Check if dir here ('.') -> return file
+  if empty(a:dir) || a:dir =~# '^\.[/\\]\?$'
+    return a:file
+  endif
   let result = []
   if vimwiki#u#is_windows()
     " TODO temporary fix see #478