RFR: 2588: Auto Labeling should ignore merge commits

[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.

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Commit messages:
 - rename
 - SKARA-2588

Changes: https://git.openjdk.org/skara/pull/1740/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1740&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2588
  Stats: 239 lines in 4 files changed: 157 ins; 65 del; 17 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