A page titled CharacterPrefix-2012-03-18 confuses s:group_links() in diary.vim when diary_rel_path = the empty string

What steps will reproduce the problem?
1. set 'diary_rel_path' = the empty string
2. create a new diary note

... you can probably simulate this error by placing such a file in the diary 
directory (if your diary_rel_path is not the empty string)

What is the expected output? What do you see instead?

Instead of seeing the new note, an error message appears:

 > Error detected while processing function vimwiki#diary#make_note..<SNR>73_make_date_link..<SNR>73_add_link..<SNR>73_format_diary..<SNR>73_group_links:
 > line   14:
 > E716: Key not present in Dictionary: So
 > E716: Key not present in Dictionary: yX
 > E716: Key not present in Dictionary: ar

This is because line 114 in diary.vim obtains two characters instead of the 
numeric month:
    let month = strpart(fl, 5, 2)

For example, with a page named "CharacterPrefix-2012-03-18.md", the variable 
month would be set equal to 'ct'.

Despite complaining, s:group_links() actually creates an interesting diary 
index file !!

I am using 1.3dev, rd2d7783ec0ee (2012-03-18).

Original issue reported on code.google.com by stu.andrews on 18 Mar 2012 at 8:03

vimwiki-backup / vimwiki

A page titled CharacterPrefix-2012-03-18 confuses s:group_links() in diary.vim when diary_rel_path = the empty string #291