RFR: Allow merge PRs with an exact hash

[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.

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Commit messages:
 - Allow merge PRs with an exact hash

Changes: https://git.openjdk.java.net/skara/pull/1026/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1026&range=00
  Stats: 164 lines in 2 files changed: 163 ins; 1 del; 0 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