Dependency on generated item always unmet

[denisdefreyne]

Steps to reproduce:

1. Create a new site

2. Add the preprocess rule:

   preprocess do
     items.create('Hallo', {}, '/hallo.md')
   end

3. Add <p><%= @items['/hallo.*'].compiled_content %></p> to content/index.html

4. Update the index compilation rule:

   compile '/**/*.html' do
     filter :erb
     layout '/default.*'
   end

Expected result: the /index.html page is not needlessly recompiled.
Actual result: the /index.html page is always recompiled.

[denisdefreyne]

Fix in #893.