Use find_or_create_branch in BranchCreation

This method was added after the class was created, but it makes sense to use here. If the branch already exists, we just want to return that branch, rather than returning nil via ignoring_duplicates.

9f2f204c - Move ignoring_duplicates into MonthlyPreparationService

It's now only used for creating the monthly Pick into label in an idempotent way. This method being available as a by-product of including the BranchCreation module was a poor design decision to begin with.

Closes gitlab-com/gl-infra/delivery#1014 (closed)