RFR: pr: only create backport branch once

[Robin Westberg]

On Wed, 31 Mar 2021 13:57:33 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that ensures that the `/backport` commit command will only create the backport commit (and corresponding branch) once. There is no need to try to do this multiple times, the cherry picking will result in the same result every time. If a user for some reasons issues the `/backport` command twice on a commit then this will now work.
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/1107