pre-upload: Add comment to --ignore=GERRIT_CHANGE_ID option

This check for Change-Id: is present in checkpatch.pl since kernel 3.15,
introduced in commit 7ebd05ef1646e8cbef54e38343722741a4744626 upstream.

For upstream kernel, Change-Id: is noise, but as we do use Gerrit to
track commits in all our repos, it is actually required for us.
Disable the checkpatch.pl check for it to suppress the warning.

BUG=chromium:533701
TEST=Presubmit passes.

Change-Id: Iec8d9dd4ec0b3c95eb9559d95d01996757840055
Signed-off-by: Filipe Brandenburger <filbranden@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/304866
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/pre-upload.py b/pre-upload.py
index 11b51fe..1d2c5b1 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -1098,6 +1098,9 @@
   # information on that file in our source trees, so let's suppress the
   # warning.
   options.append('--ignore=FILE_PATH_CHANGES')
+  # Do not complain about the Change-Id: fields, since we use Gerrit.
+  # Upstream does not want those lines (since they do not use Gerrit), but
+  # we always do, so disable the check globally.
   options.append('--ignore=GERRIT_CHANGE_ID')
   cmd = ['%s/checkpatch.pl' % hooks_dir] + options + ['-']
   cmd_result = cros_build_lib.RunCommand(cmd=cmd,