meta: modify pull request template for prepending

The GitHub interface for new pull requests has started prepending
commit messages for single-commit pull requests. It had previously
been appending them. Modify the GitHub template to accommodate the new
behavior.

PR-URL: https://github.com/nodejs/node/pull/10484


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>