RFR: 2115: More flexible merge PR review configuration [v2]

[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

Erik Duveblad has updated the pull request incrementally with one additional commit since the last revision:

  Review feedback

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Changes:
  - all: https://git.openjdk.org/skara/pull/1592/files
  - new: https://git.openjdk.org/skara/pull/1592/files/9e653ecf..4303440b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1592&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1592&range=00-01

  Stats: 9 lines in 3 files changed: 6 ins; 0 del; 3 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