roll-dep: Add comments indicating that --no-log needs to keep working

Bug:
Change-Id: Id5efd1e84318b95297c6d83f77e58f617d7db8de
Reviewed-on: https://chromium-review.googlesource.com/690242
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>

roll_dep.py

@@ -145,6 +145,8 @@ def roll(root, deps_dir, roll_to, key, reviewers, bug, no_log, log_limit,
     log_section = log_url + '\n\n'
   log_section += '$ %s ' % ' '.join(cmd)
   log_section += '--format=\'%ad %ae %s\'\n'
+  # It is important that --no-log continues to work, as it is used by
+  # internal -> external rollers. Please do not remove or break it.
   if not no_log and should_show_log(upstream_url):
     if len(cleaned_lines) > log_limit:
       # Keep the first N log entries.
@@ -187,6 +189,8 @@ def main():
       help='To specify multiple reviewers, use comma separated list, e.g. '
            '-r joe,jane,john. Defaults to @chromium.org')
   parser.add_argument('-b', '--bug', help='Associate a bug number to the roll')
+  # It is important that --no-log continues to work, as it is used by
+  # internal -> external rollers. Please do not remove or break it.
   parser.add_argument(
       '--no-log', action='store_true',
       help='Do not include the short log in the commit message')