Improve clang-format-diff help output

The description in clang-format-diff.py is more useful than the one
in `clang-format-diff -h`, so use the same description in both places.

Differential Revision: https://reviews.llvm.org/D64998

git-svn-id: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@366828 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
1 file changed