Tools: prepare-commit-msg.js is prepending "[not verified]" to commits after rebase.

I just rebased #16699 from `master`, and noticed after I pushed it that `[not verified]` has been prepended to every commit message. It seems this message is coming from `prepare-commit-msg.js`.

I suspect that something has changed in the latest version of the GitHub desktop app: this is the first rebase I've done since upgrading it, and I'm not aware of anything else changing that might cause this.

I've marked this issue as High priority, since some quick testing shows that the next rebase will prepend an additional `[not verified]` to all of the commits already marked with `[not verified]`, and this is going to get out of hand very quickly.

Additionally, I'd appreciate if someone more well versed in CLI git than I could come up with a command to undo the damage.

#### Steps to reproduce the issue

All of these steps should be performed in the GitHub desktop app.

1. Create a new branch off of `master`.
2. Make some changes, and commit them. (I don't know if you also need to push the changes.)
3. Open the Branch menu, select "Rebase current branch...", and click "Start rebase".
4. Observe `[not verified]` being prepended to the commit message.

#### What I expected

For my commit messages to not be modified.

#### What happened instead

My commit messages were modified.

@Automattic/jetpack-crew

Start with prepare-commit-msg.js and reproduce the listed rebase flow in the GitHub Desktop app, comparing commit messages before and after rebasing. Done means rebasing no longer prepends repeated "[not verified]" text to commit messages.

Written by the indexing model from the issue text.