gerrit: add dryrun for private/public helper

This fixes -n handling with `gerrit private ...`.

BUG=None
TEST=`gerrit -n private 12345 yes` no longer attempts to update the CL

Change-Id: If3734eb22f5c67a51eb265ac5bc0552270289259
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2044495
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/gerrit.py b/scripts/gerrit.py
index 9ca4788..84a26d8 100644
--- a/scripts/gerrit.py
+++ b/scripts/gerrit.py
@@ -502,7 +502,7 @@
     raise RuntimeError('Unknown "boolean" value: %s' % private_str)
 
   helper, cl = GetGerrit(opts, cl)
-  helper.SetPrivate(cl, private)
+  helper.SetPrivate(cl, private, dryrun=opts.dryrun)
 UserActPrivate.usage = '<CL> <private str>'