fixes failure when a commit has already been cherry-picked

cherrypick_cl.py fails whenever a commit is already present, but the
revert checker should simply note this and move on.

BUG=chromium:1181376
TEST=nightly_revert_checker_test.py

Change-Id: I04500fef98a95607df3cb23bf0a901d3ba5062a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2716971
Auto-Submit: Christopher Di Bella <cjdb@google.com>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
3 files changed