Dist-git MR's target branch to match source-git MR's target branch

[csomh]

When hardly creates an MR in dist-git, the target branch for the MR should be the same as the target branch of the source-git MR. (Currently this is always set to default branch of the repository). Example: rawhide → rawhide.

A new configuration key in .distro/source-git.yaml, called downstream_branch_name, should be used to override this behavior (if specified), and allow explicitly setting the name of the target branch in dist-git. Example: fedora-rawhide → rawhide.

In case the target branch does not exist in dist-git, a comment informing about this should be made on the source-git MR.

[jpopelka]

Currently this is always set to c9s

Do you know where is it set? I can't find it.

[csomh]

It's actually the default branch of the dist-git repository, which in case of gitlab.com/redhat/centos-stream is c9s. This could be customized by setting the dist_git_branch argument when sync_release is called in hardly.