RFR: vcs: introduce "Backport-of" trailer

[Robin Westberg]

On Wed, 30 Sep 2020 13:45:34 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> 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

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

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