git-cl: fix auth error message computation

Bug:
Change-Id: Ic57563b131ff7e50985748eeb3d55e4b69b8db87
Reviewed-on: https://chromium-review.googlesource.com/562279
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

git_cl.py

@@ -2438,8 +2438,8 @@ class _GerritChangelistImpl(_ChangelistCodereviewBase):
       return
     else:
       missing = (
-          [] if gerrit_auth else [self._gerrit_host] +
-          [] if git_auth else [git_host])
+          ([] if gerrit_auth else [self._gerrit_host]) +
+          ([] if git_auth else [git_host]))
       DieWithError('Credentials for the following hosts are required:\n'
                    '  %s\n'
                    'These are read from %s (or legacy %s)\n'