nanoc: Small chance to compile wrong page with ruby 2.7

Under Ruby 2.7 there is a very small chance that nanoc compile will use the wrong content for a page when a large number of pages change at the same time

Steps to reproduce

1. use ruby-2.7
2. Make a small change that affects all pages, like adding a single character to the layout
3. nanoc compile
4. Clear the cache to force a recompile rm -rf tmp
5. nanoc compile --diff

Expected behavior

The second compile should show no changes.

Actual behavior

About 2 to 4 of every 1000 pages changes, to the correct version. Subsequent nanoc compile commands show no changes.

Details

It seems to work OK on ruby 2.6 and it also works fine on smaller repos and smaller changes. Only during a compile that affects 1000 pages does the compilation get confused.