Auto merge of #2436 - dojoteef:reuse_preview_buffer, r=micbou

Address #1366 by reusing a single preview buffer

* Additionally set the preview buffer to be unlisted so it does not interfere with commands like :bnext
* See [my comment](https://github.com/Valloric/YouCompleteMe/issues/1366#issuecomment-259878581) for more details

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python/ycm/tests/vimsupport_test.py

@@ -1262,6 +1262,8 @@ def WriteToPreviewWindow_test( vim_current, vim_command ):
     call( 'modifiable', True ),
     call( 'readonly', False ),
     call( 'buftype', 'nofile' ),
+    call( 'bufhidden', 'wipe' ),
+    call( 'buflisted', False ),
     call( 'swapfile', False ),
     call( 'modifiable', False ),
     call( 'modified', False ),

python/ycm/vimsupport.py

@@ -919,6 +919,8 @@ def WriteToPreviewWindow( message ):
     vim.current.buffer[:] = message.splitlines()
 
     vim.current.buffer.options[ 'buftype' ]    = 'nofile'
+    vim.current.buffer.options[ 'bufhidden' ]  = 'wipe'
+    vim.current.buffer.options[ 'buflisted' ]  = False
     vim.current.buffer.options[ 'swapfile' ]   = False
     vim.current.buffer.options[ 'modifiable' ] = False
     vim.current.buffer.options[ 'readonly' ]   = True