RFR: 2591: Improve auto labeler

[Zhao Song]

After [SKARA-2588](https://bugs.openjdk.org/browse/SKARA-2588) is deployed, users are complaining about the behavior of auto labeler.

After discussion with Erik, we propose the following behavior for the auto labeler:
 1. The auto labeler will never remove labels that have already been added.
 2. The only case in which the auto labeler upgrades labels to group labels is when it detects a bunch of new labels to add. In this case, it will attempt to update the labels to group labels.
 3. Users can add any labels they want.
 Example:
 1. In the initial labeling, the auto labeler adds “hotspot-runtime” to the PR.
      (Current labels: “hotspot-runtime”)
 2. The user pushes three commits that touch “hotspot-gc” and “hotspot-jfr”. The auto labeler upgrades these to the group label “hotspot” and adds it to the PR.
     (Current labels: “hotspot-runtime”, “hotspot”)
3. Later, the user pushes a new commit touches only “hotspot-gc”. The auto labeler will not add “hotspot-gc” to the PR since “hotspot” label is already there.
     (Current labels: “hotspot-runtime”, “hotspot”)

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Commit messages:
 - SKARA-2591

Changes: https://git.openjdk.org/skara/pull/1741/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1741&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2591
  Stats: 100 lines in 4 files changed: 32 ins; 32 del; 36 mod
  Patch: https://git.openjdk.org/skara/pull/1741.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1741/head:pull/1741

PR: https://git.openjdk.org/skara/pull/1741