Aggregate labels from merged pull requests

[ohbarye]

@onk @motemen

Issue

Closes #54

Changes

As described in https://github.com/x-motemen/git-pr-release/issues/54, I added aggregate_labels feature.

• Add pr-release.aggregate_labels config (or GIT_RELEASE_PR_AGGREGATE_LABELS).
  • It accepts boolean-ish value: "true" or anything
  • Only when 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"].

Testing

That worked well on my own

$ GIT_PR_RELEASE_BRANCH_STAGING=release GIT_PR_RELEASE_LABELS=release GIT_PR_RELEASE_AGGREGATE_LABELS=true bundle exec git-pr-release

Merged PRs have labels	Created PR has labels

Review Points

• Is aggregate_labels an appropriate name?
  • ideas: reuse_labels, summarize_labels... etc.
• Should aggregate_labels be given as a command line arg, not environment variable or git config?
  • As like --dry-run, --no-fecth, aggregate_labels is boolean-ish value. So I wonder if it should be. (It depends on this gem's command line design.)