Backport #21720.
Fix#20921.
The `ctx.Repo.GitRepo` has been used in deleting issues when the issue
is a PR.
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>