Summary:
In a previous commit, I changed the way to checkout release branches from "git checkout <branch_name>" to "git checkout origin/<branch_name>". However, this doesn't seem to work in our CI environment. Revert it.
Closes https://github.com/facebook/rocksdb/pull/2189

Differential Revision: D4922294

Pulled By: siying

fbshipit-source-id: 482c17f9b05e6ccb190876b050682fe5a458103d