Skip branch creation on PR saga retry

[adboio]

TL;DR

Prevent duplicate branch creation attempts when the PR saga is retried by checking if the target branch already exists before attempting to create it.

closes #1443

Problem

When the PR creation saga is retried, it was attempting to create a branch that may have already been created in the previous attempt, causing unnecessary operations and potential failures.

Changes

• Added a check to compare the current branch against the target branch name before executing branch creation
• Only create the branch if the current branch differs from the target branch name (indicating it hasn't been created yet)
• On retry scenarios, the branch creation step is skipped since we're already on the desired branch
• Refactored originalBranch variable to currentBranch for clarity since we check it at the start

How did you test this?

This is a defensive fix for the retry path in PR creation. The logic ensures that if a saga is retried after branch creation has already succeeded, we won't attempt to recreate the branch.