[clang-format] Fix lines regression in clang-format.py

Summary:
This patch fixes a regression after https://reviews.llvm.org/rL305665,
which updates the structure of the `lines` variable.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D37011

llvm-svn: 311456