Normalize buffer filepath

On Windows, Vim returns buffer filepaths with forward slashes instead of
backward ones if the shellslash option is set. Since we always expect backward
slashes from the server, we normalize these paths.

python/ycm/vimsupport.py

@@ -158,7 +158,7 @@ def BufferIsVisible( buffer_number ):
 
 def GetBufferFilepath( buffer_object ):
   if buffer_object.name:
-    return ToUnicode( buffer_object.name )
+    return os.path.normpath( ToUnicode( buffer_object.name ) )
   # Buffers that have just been created by a command like :enew don't have any
   # buffer name so we use the buffer number for that.
   return os.path.join( GetCurrentDirectory(), str( buffer_object.number ) )