[AINFRA-1406] Fix deletion of intermediate branch if nothing to backmerge

[AliSoftware]

What does it do?

Fixes the issue during create_release_backmerge_pull_request when there is nothing to backmerge but the automation already created the intermediate merge/… backmerge branch and is now trying to delete that branch to cancel the process and clean things up.

So far in that specific case we were still checked out on the merge/… intermediate when we were trying to delete that branch, but git doesn't allow to delete the current branch:

[!]     Error creating backmerge pull request:
--
  |  
  | ```
  | git '--git-dir=/opt/ci/builds/builder/automattic/day-one-ios/.git' '--work-tree=/opt/ci/builds/builder/automattic/day-one-ios' '-c' 'core.quotePath=true' '-c' 'color.ui=false' 'branch' '-D' 'merge/release-2025.21-into-trunk'  2>&1
  | status: pid 2243 exit 1
  | output: "error: cannot delete branch 'merge/release-2025.21-into-trunk' used by worktree at '/opt/ci/builds/builder/automattic/day-one-ios'\n"

The fix is to git checkout the previous branch (the head branch we were potentially trying to backmerge) before deleting the intermediate branch, so that we're sure we're not in the branch we're deleting.

Checklist before requesting a review

• Run bundle exec rubocop to test for code style violations and recommendations.
• Add Unit Tests (aka specs/*_spec.rb) if applicable.
• Run bundle exec rspec to run the whole test suite and ensure all your tests pass.
• Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section.
• If applicable, add an entry in the MIGRATION.md file to describe how the changes will affect the migration from the previous major version and what the clients will need to change and consider.

[iangmaia]

👍