/usr/bin/clang-tidy-diff-6.0.py is in clang-tidy-6.0 1:6.0-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/../lib/llvm-6.0/share/clang/clang-tidy-diff.py
/usr/bin/../lib/llvm-6.0/share/clang/clang-tidy-diff.py