RFR: 1855: Make it configurable which repositories merges should be allowed from

[Zhao Song]

In this patch, PR bot has been added with the capability to control which repositories can be the source for Merge-style pull requests.

By default, all repositories are allowed.

However, if it is desired to configure "allowed" repos, the configuration can be added to the PR bot configuration as follows:

{
  "pr": {
      "repositories": {
          "repo1": {
               "mergeSources": [openjdk/repo2, openjdk/repo3]
          }
       }
  }
}


And the repo itself can always be the source of Merge-style pull requests.

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

Commit messages:
 - fix a problem
 - add factory tests
 - SKARA-1855

Changes: https://git.openjdk.org/skara/pull/1503/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1503&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1855
  Stats: 88 lines in 7 files changed: 83 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/skara/pull/1503.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1503/head:pull/1503

PR: https://git.openjdk.org/skara/pull/1503