RFR: Allow merge PRs with an exact hash [v2]

[Robin Westberg]

> Make it possible to create a merge-style PR with commits that are not already present in another existing repository. In this case, the hash of the commit to be merged must be specified, to allow for proper rebase / squash of the merge commit itself.

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Allow configuration of integrators who can create merge-style pull requests

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1026/files
  - new: https://git.openjdk.java.net/skara/pull/1026/files/6d9ea629..cce9630f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1026&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1026&range=00-01

  Stats: 154 lines in 5 files changed: 142 ins; 5 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/1026.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1026/head:pull/1026

PR: https://git.openjdk.java.net/skara/pull/1026