chore: merge forward script fails when branch is already checked out (#…

…10946)

The [AutoPullRequest] CodeBuild job already checks out the base branch.
This causes the `git checkout -b` command to fail since the branch
already exists.

Update the script so that branch creation occurs only if it does not
already exist.

[AutoPullRequest]: https://github.com/aws/cdk-ops/blob/21acd7ff37ad9ab14d8c3f9b41980e9f62f64503/lib/cdk-pipeline.ts#L284-L290


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

scripts/merge-forward.sh

@@ -4,5 +4,5 @@
 set -exo pipefail
 
 git fetch --all
-git checkout -b v2-main origin/v2-main
-git merge origin/master --no-edit
+git checkout -B v2-main origin/v2-main
+git merge origin/master --no-edit