RFR: 1860: Make it possible to configure PR bot to jcheck all commits in "Merge PR"

[Zhao Song]

In this patch, PR bot has been added with the capability to control whether jcheck every commit in a merge PR.

By default, jcheckMerge is enabled.

However, if it is desired to enable jcheckMerge in a repo, the configuration can be added to the PR bot configuration as follows:

{
  "pr": {
      "repositories": {
          "repo1": {
               "jcheckMerge": true
          }
       }
  }
}

If jcheckMerge is true, then the pr bot would jcheck all the commits in the merge PR and if any jcheck doesn't pass, the error message will be treated as Integration blocker.

-------------

Commit messages:
 - SKARA-1860

Changes: https://git.openjdk.org/skara/pull/1510/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1510&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1860
  Stats: 130 lines in 8 files changed: 112 ins; 5 del; 13 mod
  Patch: https://git.openjdk.org/skara/pull/1510.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1510/head:pull/1510

PR: https://git.openjdk.org/skara/pull/1510