Site not rebuilding if the latest commit was amended

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [x] The site generated by GitHub Pages
- [ ] Building sites locally

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

### What did you do (e.g., steps to reproduce)
I amended the last commit using the command:
git commit --amend --no-edit
then did a push force to the master.
git push origin master --force
### What did you expect to happen?
I expected GitHub Pages to rebuild the site on the amended commit. But it didn't.
### What happened instead?
It didn't
### Additional information

* Link to the live site (if applicable):
https://jonatankruszewskiitc.github.io/pre-course-FS-2020/

* Link to the source repo (if applicable):
https://github.com/jonatankruszewskiITC/pre-course-FS-2020

Il report non indica alcun file o test di pages-gem. Inizia dal repository del codice sorgente collegato e riproduci la sequenza di force-push del commit modificato; quindi confronta lo stato della build ospitata con il sito distribuito. Per considerare completato il lavoro, sarebbe necessario identificare il componente responsabile dell’attivazione della rebuild oppure confermare che il comportamento è esterno a pages-gem.

Scritto dal modello di indicizzazione a partire dal testo della issue.