Auto merge of #2889 - bstaletic:PostVimMassage_truncate_fix, r=puremourning

[READY] Fix truncation of messages when len(message) == vim_width

# PR Prelude

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- [x] I have read and understood YCM's [CONTRIBUTING][cont] document.
- [x] I have read and understood YCM's [CODE_OF_CONDUCT][code] document.
- [x] I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.
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# Why this change is necessary and useful

[Please explain **in detail** why the changes in this PR are needed.]

When the message is exactly as wide as the vim window, vim still
displays "press enter..." message.

The bug can be demonstrated if we hardcode the `message` to `&columns * 'a'` and forcing `truncate` to `True`.

No tests because I didn't think it was necessary.

[cont]: https://github.com/Valloric/YouCompleteMe/blob/master/CONTRIBUTING.md
[code]: https://github.com/Valloric/YouCompleteMe/blob/master/CODE_OF_CONDUCT.md

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python/ycm/vimsupport.py

@@ -464,7 +464,7 @@ def PostVimMessage( message, warning = True, truncate = False ):
     vim_width = GetIntValue( '&columns' )
 
     message = message.replace( '\n', ' ' )
-    if len( message ) > vim_width:
+    if len( message ) >= vim_width:
       message = message[ : vim_width - 4 ] + '...'
 
     old_ruler = GetIntValue( '&ruler' )