tmp/ nuked when output_dir is changed in preprocessor

[denisdefreyne]

Steps to reproduce

1. Create a new site

2. Create content/stuff.html with some content

3. Create the following Rules file:

   preprocess do
     @config[:output_dir] = 'ootpoot'
   end
   
   passthrough '/**/*'
   

4. Run nanoc --verbose

5. Run nanoc --verbose again

Expected behavior

• Nanoc shows skip for ootpoot/stuff.html
• The tmp/nanoc directory has some contents

Actual behavior

• Nanoc shows identical for ootpoot/stuff.html
• The tmp/nanoc directory is empty

Details

Cleanup#cleanup_unused_stores removes the tmp directory based on the hash of the output directory, after it’s been changed.

[denisdefreyne]

CC @gpakosz