[PATCH v8 18/44] branch.c: use ref transaction for all ref updates

Change create_branch to use a ref transaction when creating the new branch. ref_transaction_create will check that the ref does not already exist and fail otherwise meaning that we no longer need to keep a lock on the ref during the setup_tracking. This simplifies the code since we can now do the

Re: [PATCH v8 18/44] branch.c: use ref transaction for all ref updates

Ronnie Sahlberg wrote: Change create_branch to use a ref transaction when creating the new branch. ref_transaction_create will check that the ref does not already exist and fail otherwise meaning that we no longer need to keep a lock on the ref during the setup_tracking. This simplifies the