Conflict with automated markup links prevents adding new checklist items

[iandunn]

If a checklist item has a trigger for an automatic markup link, then no items can be added directly after it. The automatic markup links code is specific to WordPress.com, but other users could experience similar conflicts.

Steps to reproduce:

1. Create a list inside a post
2. Click on one of the entries, then on the Add New After button
3. Enter the following as the new item content and submit it: text with a trigger for automated markup links like #121-gh-wp-super-cache foo bar
4. Click on the new item from the previous step, then on the Add new After button
5. Enter this content and submit it: just regular text without anything special

The request in the final step will fail. If you try to add it after an item that doesn't have the trigger it will work. It also works if the [issue https://github.com/automattic/wp-super-cache/pull/121] shortcode is used instead of the trigger for the automated link.

[iandunn]

Just noticed that editing the item with the trigger will also fail.