Integrated: pr: introduce backport pull requests

[Erik Helin]

Hi all,

please review this patch that introduces the concepts of "backport style" pull requests. If a pull request title
matches `Backport <hash>` then issues and summaries from `<hash>` will be added to the current pull request. If the
"backport style" pull request is integrated then a `Backport-of: <hash>` trailer will also be automatically added to
the commit message.

Testing:
- [x] Added four new unit tests
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - pr: introduce backport pull requests

Changes: https://git.openjdk.java.net/skara/pull/868/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=868&range=00
  Stats: 489 lines in 7 files changed: 484 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/868.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/868/head:pull/868

PR: https://git.openjdk.java.net/skara/pull/868