Fix traceback on quote in filename

Fixes #1109

python/ycm/vimsupport.py

@@ -104,7 +104,7 @@ def GetUnsavedAndCurrentBufferData():
 
 def GetBufferNumberForFilename( filename, open_file_if_needed = True ):
   return GetIntValue( u"bufnr('{0}', {1})".format(
-      os.path.realpath( filename ),
+      EscapeForVim( os.path.realpath( filename ) ),
       int( open_file_if_needed ) ) )