RFR: vcs: introduce "Backport-of" trailer

[Erik Helin]

Hi all,

please review this pull request that adds support for the "Backport-of" trailer to commit messages.

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

Thanks,
Erik

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Commit messages:
 - vcs: introduce "Backport-of" trailer

Changes: https://git.openjdk.java.net/skara/pull/856/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=856&range=00
  Stats: 91 lines in 8 files changed: 85 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/856.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/856/head:pull/856

PR: https://git.openjdk.java.net/skara/pull/856