[clang-tidy] Add -path option to clang-tidy-diff.py

Summary:
This flag allows specifying a custom path for the compilation
database.  Unfortunately we can't use the -p flag like other
clang-tidy tools because it's already taken.

Reviewers: alexfh

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D29806

llvm-svn: 295482