Change diary_rel_path minimum length to 0 (#689)

Changing this option to allow empty strings places the diary files in the same directory as the main wiki files. This is more backwards compatible with previous releases.

autoload/vimwiki/vars.vim

@@ -265,7 +265,7 @@ function! s:populate_wikilocal_options()
         \ 'custom_wiki2html_args': {'type': type(''), 'default': ''},
         \ 'diary_header': {'type': type(''), 'default': 'Diary', 'min_length': 1},
         \ 'diary_index': {'type': type(''), 'default': 'diary', 'min_length': 1},
-        \ 'diary_rel_path': {'type': type(''), 'default': 'diary/', 'min_length': 1},
+        \ 'diary_rel_path': {'type': type(''), 'default': 'diary/', 'min_length': 0},
         \ 'diary_caption_level': {'type': type(0), 'default': 0, 'min': -1, 'max': 6},
         \ 'diary_sort': {'type': type(''), 'default': 'desc', 'possible_values': ['asc', 'desc']},
         \ 'exclude_files': {'type': type([]), 'default': []},

doc/vimwiki.txt

@@ -3312,6 +3312,7 @@ Contributors and their Github usernames in roughly chronological order:
     - Rane Brown (@ranebrown)
     - Patrik Willard (@padowi)
     - Steve Dondley (@sdondley)
+    - Alexander Gude (@agude)
 
 
 ==============================================================================
@@ -3327,6 +3328,7 @@ https://github.com/vimwiki-backup/vimwiki/issues.
 2.5 (in progress)~
 
 New:~
+    * PR #689: Allow |vimwiki-option-diary_rel_path| to be an empty string.
     * PR #683: Improve layout and format of key binding documentation in
       README and include note about key bindings that may not work.
     * PR #675: Add option |vimwiki-option-name| to assign a per wiki name.
@@ -3665,8 +3667,6 @@ http://code.google.com/p/vimwiki/issues/list
     * Issue 218: Command to generate HTML file and open it in webbrowser. See
       |:Vimwiki2HTMLBrowse|(bind to <Leader>whh)
     * NEW: Added <Leader>wh mapping to call |:Vimwiki2HTML|
-
-
 ...
 
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