RFR: 2068: Backport command should use default branch of repo as default

[Zhao Song]

On Thu, 19 Oct 2023 17:05:48 GMT, Zhao Song <zsong at openjdk.org> wrote:

> When a user is using /backport command to create a backport and the user doesn't specify a target branch, skara bot would use "master" as a default target branch. However, in some repos, like lilliput-jdk17u and lilliput-jdk21u, the default branch is not "master". Therefore, it will be better to use the repository's default branch as the default target branch of /backport command.
> 
> As Erik said, the value of default branch is already available in the repository JSON. So we just need to implement a `getDefaultBranchName` method.

Thanks for the review!

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PR Comment: https://git.openjdk.org/skara/pull/1572#issuecomment-1771645722