cherry-picker ends up in bad state if a `backport-...` branch already exists

[Jackenmen]

If, for whatever reason, the backport-... branch already exists, cherry-picker ends up in a bad state:

λ python -m cherry_picker e5ee6a87a3b6fbf1a7e3856248885811f04d1f89 3.4
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Now backporting 'e5ee6a87a3b6fbf1a7e3856248885811f04d1f89' into '3.4'
Error checking out the branch backport-e5ee6a8-3.4.
fatal: A branch named 'backport-e5ee6a8-3.4' already exists.

λ python -m cherry_picker e5ee6a87a3b6fbf1a7e3856248885811f04d1f89 3.4
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Usage: __main__.py [OPTIONS] [COMMIT_SHA1] [BRANCHES]...
Try '__main__.py -h' for help.

Error: Run state cherry-picker.state=BACKPORT_LOOP_START in Git config is not known.
Perhaps it has been set by a newer version of cherry-picker. Try upgrading.
Valid states are: BACKPORT_PAUSED, UNSET. If this looks suspicious, raise an issue at https://github.com/python/core-workflow/issues/new.
As the last resort you can reset the runtime state stored in Git config using the following command: `git config --local --remove-section cherry-picker`