Cloning rollout prevents editing branch names

[data-sync-user]

Reported by a user:

I'm having trouble testing my Nimbus rollout.

1. I cloned a Nimbus A/B experiment. Its branches: tcp-off and tcp-on.
2. I changed the new experiment to a single-branched rollout.
3. I changed the single branch's name from tcp-off to tcp-on.
4. The Experimenter UI complained that my single branch had the same name as the B branch from the cloned experiment (tcp-on).
5. I worked around that by re-enabling the B branch, renaming it to tcp-off, renaming the A branch to tcp-on, and then switching back to a single branch rollout. Everything thing seems fine. My single branch is now called tcp-on.
6. I launched the rollout to preview and I can see the preview rollout's name in my Fenix's secret settings.
7. But the rollout's single branch has the old A branch name from the cloned experiment: tcp-off. I haven't tested whether this misnamed branch has the correct feature flag settings or not. That seems like an unproductive use of my time because something is clearly mixed up. (And it may be me :slightly_smiling_face: )

https://experimenter.services.mozilla.com/nimbus/total-cookie-protection-nightly-experiment

┆Issue is synchronized with this Jira Bug