[llvm-dev] merge_guards_bot reports clang-tidy/clang-format findings unrelated to modified code

[Fangrui Song via llvm-dev]

First, I love this bot:) It makes LLVM's "pushing to master" practise less awful:)
Now the main topic...

> clang-tidy: fail. Please fix clang-tidy findings.

For example, on https://reviews.llvm.org/D72103#1801916 ,
merge_guards_bot reports clang-tidy findings of existing code, not just
the modified code.

I think https://github.com/google/llvm-premerge-checks should use
--line-filter= as clang-tidy/tool/clang-tidy-diff.py does.

> clang-format: fail. Please format your changes with clang-format by running git-clang-format HEAD^ or applying this patch.

Similarly, clang-format should use --lines to filter out untouched code.