RFR: git-pr-create: support --backport option

[Robin Westberg]

On Thu, 8 Oct 2020 10:50:18 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that adds the `--backport=HASH` option to `git pr create` to make it a bit easier to create a
> "backport style" pull request from the CLI. A backporter can now only has to run:
> $ git checkout -b backport-8123456
> $ git cherry-pick <HASH>
> $ git pr create --backport=<HASH>
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/871