RFR: 2588: Auto Labeling should ignore merge commits [v2]

[Zhao Song]

> Currently, the auto labeling doesn't ignore merge commits, so when user merge master back into their feature branch, it's possible that unrelated labels are added to the pull request.
> This patch is trying to let the bot ignore all the merge commits and the commits brought in by the merges.

Zhao Song has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  SKARA-2588

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Changes:
  - all: https://git.openjdk.org/skara/pull/1740/files
  - new: https://git.openjdk.org/skara/pull/1740/files/288587ea..3880af63

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1740&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1740&range=00-01

  Stats: 38 lines in 2 files changed: 6 ins; 9 del; 23 mod
  Patch: https://git.openjdk.org/skara/pull/1740.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1740/head:pull/1740

PR: https://git.openjdk.org/skara/pull/1740