Only exclude refs/meta/config for showing tree status

The example in the doc will exclude everything:
https://chromium.googlesource.com/infra/gerrit-plugins/chumpdetector/+/HEAD/README.md

Change-Id: Idc5c722330f247065986225da02ebd4600fb6968
Reviewed-on: https://chromium-review.googlesource.com/c/infra/+/5660540
Reviewed-by: Gavin Mak <gavinmak@google.com>
1 file changed
tree: 128faf95520f758d76bb4f197e43208efc77f7b1
  1. chumpdetector.config
  2. groups
  3. project.config