When closing the pull-request, an automatic merge is made. Currently, the generated comment is not really useful as it assert:

Merge commit '12345678900' into master.

(or something like this) The most valuable information lost is the name of the branch which was merged. This is the only place where branch names are kept forlong time. And sometimes, it is usefull to have this information as it avoid to read the whole commit set to understand the meaning of the merged branch.

Furthermore, it could be really useful to let the integrator change the merge comment before validating the pull-request. Such feature is present on GitHub and GitLab. And/or embed the title+description of the pull-request into the commit merge comment.