Summary:
My previous change to use lib2to3 to migrate clang-format-diff.py
for Python 2 only works if there's nothing to reformat. Instead, give
instructions to download to REPO_ROOT.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7603

Test Plan: Try the instructions on a fresh CentOS 8 devserver

Reviewed By: riversand963

Differential Revision: D24569608

Pulled By: pdillinger

fbshipit-source-id: 1410ba163e016b226e883dec93fae3df9ed0eab2