reload items after precompilation

use case:
```ruby
preprocess do
  AutoHelper.new(File.dirname(__FILE__), self).auto # do some automated content genration

  # force reload of items
  self.site.instance_variable_set(:@loaded, false)
  self.site.instance_variable_set(:@items_loaded, false)
  self.site.load
end
```
the code above does generate warning:

> WARNING: A preprocess block is already defined. Defining another preprocess block overrides the previously one.

the proposed fix is free of this warning

no tests yet, please let me know if you like it and I can think of something

This is definitely a good change. If you can extract this into a branch on `release-3.7.x` that’d be great!