Make `git cl format --python -full --dry-run` return non-zero exit code if
files are not well formatted.

Make `git cl format --python -full --diff` print diff, rather than new file
contents.

Change-Id: I666d360051563898d3508d776817bf9a61433cb0
Reviewed-on: https://chromium-review.googlesource.com/1081340Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>