RFR: 2197: Make the /backport command support a "repo:branch" argument

[Zhao Song]

On Fri, 15 Mar 2024 18:47:16 GMT, Zhao Song <zsong at openjdk.org> wrote:

> This patch is trying to make /backport command support `repo:branch` argument.
> 
> And as ErikD suggested, `repo` should be optional if the user is backporting to a different branch in the same repository.
> 
>> To make it slightly more convenient to backport a commit to a branch in the same repository as the commit is residing in we would allow short form "/backport :branch" (this is also fully backwards compatible and cannot become ambiguous in the future).

Thanks for the review!

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PR Comment: https://git.openjdk.org/skara/pull/1620#issuecomment-2004789871