RFR: 2115: More flexible merge PR review configuration

[Erik Duveblad]

Hi all,

please review this patch that enables a bit more flexible configuration for checking merge pull requests. Today we can configure that a merge pull requests always should be checked and if that isn't configured then the `.jcheck/conf` from the repository is used. If we ever want to set `merge=check` in the `.jcheck/conf` file in repos then we will need a way to _disable_ running jcheck on merge pull requests (for example for projects using merge pull requests to sync in commits).

This patch makes it possible to configure that merge pull requests should either always be checked, never be checked or checked according to `.jcheck/conf` in the repo.

Thanks,
Erik

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

Commit messages:
 - skara-2115

Changes: https://git.openjdk.org/skara/pull/1592/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1592&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2115
  Stats: 61 lines in 11 files changed: 41 ins; 0 del; 20 mod
  Patch: https://git.openjdk.org/skara/pull/1592.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1592/head:pull/1592

PR: https://git.openjdk.org/skara/pull/1592