RFR: 2591: Improve auto labeler [v2]

[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”)

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  review comment

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Changes:
  - all: https://git.openjdk.org/skara/pull/1741/files
  - new: https://git.openjdk.org/skara/pull/1741/files/58223aa7..4edb400a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1741&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1741&range=00-01

  Stats: 124 lines in 7 files changed: 92 ins; 17 del; 15 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