RFR: 1797: Add support for /backport pull request command [v6]

[Zhao Song]

On Mon, 6 Feb 2023 21:47:02 GMT, Zhao Song <zsong at openjdk.org> wrote:

>> This patch introduces support for the /backport command in pull requests. (ErikJ and ErikH proposed the idea and came up with this implementation.)
>> 
>> Usage of /backport in pull request:
>> 
>> Syntax: `/backport <repo> [<branch>] ` ` /backport disable <repo> [<branch>]` (default branch is master)
>> 
>> 1. If the `/backport` command is used in an **open** pull request, it adds a label `Backport=repo:branch` to the PR. If the PR is later integrated, the PR bot scans for backport labels, creates the backport branch and provides a link for creating the backport. To cancel the backport, the user can use `/backport disable repo master` to remove the label before the PR is integrated.
>> 2.  If the `/backport` command is used in an **integrated** PR, it creates the backport branch and comments on the PR. This process is similar to using the /backport command in commits. (This feature was implemented in [SKARA-1495](https://bugs.openjdk.org/browse/SKARA-1495) by guoxiong li, but the usage was not documented on the[ wiki page](https://wiki.openjdk.org/display/SKARA/Pull+Request+Commands), so seems like few people use this command)
>> 3.  If the `/backport` command is used in a **closed** (not integrated) PR, the user will receive an error message stating that the command cannot be used in a closed but not integrated PR.
>
> Zhao Song has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix an issue

Thanks for review!

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PR: https://git.openjdk.org/skara/pull/1466