Default squash_commit_template of %{title} loses information in many cases

I'm duplicating this report from T366624 - GitLab's default commit templates often lose the text of existing commit messages on phabricator.wikimedia.org.

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Problem: GitLab has a concept of commit templates. The default, especially for squashed MRs, is bad.

Docs:

https://gitlab.wikimedia.org/help/user/project/merge_requests/commit_templates

The default message for squashed commits, in its entirety:

%{title}

This is just the title of the merge request, and it tends to lose nearly all of the information in commit bodies. This results in a less useful history and discourages our standard commit message practices.

It seems like it would be better for all users if this defaulted to something like %{all_commits}. (Or possibly something including title, MR description, and commits.)