RFR: 1797: Add support for /backport pull request command

[David Holmes]

On 2/02/2023 7:02 am, Zhao Song 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 and begins the backporting process. To cancel the backport, the user can use `/backport disable repo master` to remove the label before the PR is integrated.

This does not sound desirable. Fixes should undergo some bake time in 
the primary repo before they get backported anywhere. With the current 
situation it is up to the person doing the backport to go to the commit 
in question and know that the commit has been suitable tested/baked.

David
-----

> 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.
> 
> -------------
> 
> Commit messages:
>   - fix test
>   - SKARA-1797
> 
> Changes: https://git.openjdk.org/skara/pull/1466/files
>   Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1466&range=00
>    Issue: https://bugs.openjdk.org/browse/SKARA-1797
>    Stats: 299 lines in 5 files changed: 284 ins; 3 del; 12 mod
>    Patch: https://git.openjdk.org/skara/pull/1466.diff
>    Fetch: git fetch https://git.openjdk.org/skara pull/1466/head:pull/1466
> 
> PR: https://git.openjdk.org/skara/pull/1466