presubmit_diff always tries to decode files from Gitiles with UTF-8
but if the file is binary, it gets a UnicodeDecodeError. Don't
assume that a file is text and just write the bytes directly.
Bug: b/358175830
Change-Id: Iaa259051b4157737397933e9d994c00e9da5836c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5771938
Reviewed-by: Scott Lee <ddoman@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>