Revert rL356547 : [clang-tidy] Cosmetic fix

Differential Revision: https://reviews.llvm.org/D57662
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[clang-tidy] Parallelize clang-tidy-diff.py

This patch has 2 rationales:

- large patches lead to long command lines and often cause max command line length restrictions imposed by OS;
- clang-tidy runs on modified files are independent and can be done in parallel, the same as done for run-clang-tidy.

Differential Revision: https://reviews.llvm.org/D57662
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Reverted to fix buildbot: http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/45547/steps/test/logs/stdio

llvm-svn: 356553