[Proposal] Aggregate labels from pull requests

[ohbarye]

Hello maintainers. @motemen @onk

If possible, I hope this gem would have a feature that determines labels to be added to a created pull request by aggregating labels on merged pull requests.

Feature

Let's say we have 3 merged pull requests as follows.

1. a pull request with a label: ["enhancement"]
2. a pull request with labels: ["bug", "security"]
3. a pull request without any label: []

Then, a pull request that git-release-pr creates would have labels: ["enhancement", "bug", "security"].

Implementation design

• Add pr-release.aggregate_labels config.
  • It accepts boolean-ish value: "true" or "false"
  • The default is "false"
  • If it is "true", git-release-pr aggregates labels from merged pull requests.
• What if pr-release.labels is given at the same time?
  • Merge aggregated labels and pr-release.labels setting.
    • pr-release.labels=foo,bar and aggregated labels are baz, then labels would be ["foo", "bar", "baz"].

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If it's acceptable for this gem, I'm willing to make a pull request for this feature.

[onk]

It looks reasonable and the design looks good 😃