[git_cl] Append extra credential helper option

Bug: b/348024314
Change-Id: Ic7e1682812edcac878565bd79f002b40c018d95c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5655833
Commit-Queue: Allen Li <ayatane@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

git_cl.py

@@ -3627,7 +3627,10 @@ def ConfigureGitRepoAuth() -> None:
                           f'credential.{gerrit_host}.helper',
                           '',
                           modify_all=True)
-        scm.GIT.SetConfig(cwd, f'credential.{gerrit_host}.helper', 'luci')
+        scm.GIT.SetConfig(cwd,
+                          f'credential.{gerrit_host}.helper',
+                          'luci',
+                          append=True)
 
     # Override potential global gitcookie config
     scm.GIT.SetConfig(cwd, 'http.gitcookies', '', modify_all=True)