RFR: 2510: Bug closed due to inactivity even though there was "activity"

[Zhao Song]

Currently, the PullRequestPrunerBot considers only comments, reviews, and review comments as activity. Therefore, it can sometimes mistakenly close a user's pull request.

In this patch, I am trying to let PullRequestPrunerBot counts commits, pr state changes as valid activities.

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Commit messages:
 - update
 - update
 - update
 - update
 - 2510

Changes: https://git.openjdk.org/skara/pull/1723/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1723&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2510
  Stats: 135 lines in 12 files changed: 123 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/skara/pull/1723.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1723/head:pull/1723

PR: https://git.openjdk.org/skara/pull/1723