Summary:
For forward_compatible_checkout_objs the local branch is already created in previous step. This patch avoid recreating it. This should address "fatal: A branch named '3.10.fb' already exists." errors.
Closes https://github.com/facebook/rocksdb/pull/2606

Differential Revision: D5443786

Pulled By: maysamyabandeh

fbshipit-source-id: 69d5a67b87677429cf36e3a467bd114d341f3b9c