Post url updated before site is actually deployed and new urls are available in site.posts

### 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)
- Pushed a post
- Renamed the date to correctly reflect today and pushed it

### What did you expect to happen?
- The post to have the old date until a new deployment is finished.

### What happened instead?
Between pushing the ammended post and that deployment being finished, the site is partially broken:
- The ammended post is displayed in the overview with the old date and a link to the old url
- When clicking on that url, the post is not available and returns a 404
- When navigating to the new url, the post is available

### Additional information

* Link to the live site (if applicable): https://prinsfrank.nl
* Link to the source repo (if applicable): https://github.com/PrinsFrank/PrinsFrank.nl