RFR: Make the auto labeller less aggressive

[Robin Westberg]

Only run automatic labeling of PRs once, to avoid interfering with manual decisions.

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Commit messages:
 - Make the auto labeller less aggressive

Changes: https://git.openjdk.java.net/skara/pull/992/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=992&range=00
  Stats: 225 lines in 4 files changed: 162 ins; 20 del; 43 mod
  Patch: https://git.openjdk.java.net/skara/pull/992.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/992/head:pull/992

PR: https://git.openjdk.java.net/skara/pull/992