When running `arc diff` on a newly added file, utils/arcanist/clang-format.sh is run. For new files there is no change, but the script will bail. Without the use of the -f flag, we get interactive prompts like: rm: remove write-protected regular file '/tmp/tmp.ReMybrBw35'? Reviewed By: MaskRay Differential Revision: https://reviews.llvm.org/D148555