RFR: 1853: Make it possible to disable merge PRs for a repository [v3]

[Zhao Song]

> In this patch, PR bot has been added with the capability to control whether merge-style pr is allowed in specific repositories.
> 
> By default, merge-style pr is enabled.
> 
> However, if it is desired to disable merge-style pr in a repo, the configuration can be added to the PR bot configuration as follows:
> 
> {
>   "pr": {
>       "repositories": {
>           "repo1": {
>                "merge": false
>           }
>        }
>   }
> }
> 
> 
> If merge-style pr is disabled and the title of the pr matches the pattern of merge-style pr, the pr bot would reply a warning message to the user and prompt for the next steps.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  fix a problem

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1499/files
  - new: https://git.openjdk.org/skara/pull/1499/files/05244e22..922cb193

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1499&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1499&range=01-02

  Stats: 31 lines in 2 files changed: 9 ins; 19 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1499.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1499/head:pull/1499

PR: https://git.openjdk.org/skara/pull/1499