RFR: 2131: Make second jcheck run work on merge-style PR

[Zhao Song]

When testing [SKARA-2115](https://bugs.openjdk.org/browse/SKARA-2115), I find that if we change the jcheck conf in the source branch of a merge style PR. The pr will NOT run jcheck against the localJCheckConf(merged conf of source and target JCheckConf).
After investigation, I realized that this bug exists since the first time we introduced the second jcheck run feature. This bug was partly fixed by introducing IsJCheckConfUpdatedInMergePR.
However, IsJCheckConfUpdatedInMergePR will only be set correctly when workItem.bot.jcheckMerge() is true.
To fix it, we should set isJCheckConfUpdatedInMergePR correctly when the PR is a merge-style PR.

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Commit messages:
 - SKARA-2131

Changes: https://git.openjdk.org/skara/pull/1595/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1595&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2131
  Stats: 109 lines in 2 files changed: 99 ins; 8 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1595.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1595/head:pull/1595

PR: https://git.openjdk.org/skara/pull/1595