Switching to Live branch: Prevent code from executing after new code has been put in place.

[bisko]

This PR is a result of Automattic/jetpack#15209 .

After switching to another branch that's not "current", compared to the "old" branch that we switch out of, can see warnings or errors in the error log.

This happens because we continue executing actions and shutdown handlers after the code on the filesystem is changed and this causes a desync in what it's supposed to do.

This PR adds a forced exit() after we redirect the user back to WP-Admin after the new branch has been put in place and options updated to make sure those actions and handlers are not executed and no errors are triggered.

To test:

Since this relies on specific conditions to be met, generic testing is a bit hard.

For the moment it would work with the following instructions:

• Activate Bleeding edge Jetpack
• After it's activated, Activate Feature branch fix / post by email refactoring - Post by Email: Refactoring and Unit Tests jetpack#15139
• Without the patch this would trigger a Fatal error even in the WP-Admin UI
• With the patch you will be redirected back to WP-Admin and the branch activated.

[jeherve]

This seems to work well in my tests. 🚢