RFR: 2141: Inform user about accepting fork repo invite in /backport command reply

[Zhao Song]

On Thu, 11 Jan 2024 22:48:53 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> I just realized that we have the `fork.canPush` check further up. We could make this comment conditional on that as well. If the user already has push permission, we don't need to remind them of this.
> 
> In this case, we should change the message a bit.

Ah, right! I will change it.

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PR Comment: https://git.openjdk.org/skara/pull/1598#issuecomment-1888094087