Reset multi-selection in Picker:refresh() (#830)

If there's a new finder the multi selection is invalidated, so reset it

lua/telescope/pickers.lua

@@ -640,7 +640,10 @@ function Picker:refresh(finder, opts)
   if opts.reset_prompt then self:reset_prompt() end
 
   self.finder:close()
-  if finder then self.finder = finder end
+  if finder then
+      self.finder = finder
+      self._multi = MultiSelect:new()
+  end
 
   self.__on_lines(nil, nil, nil, 0, 1)
 end