Add .mailmap

We have some commits with the author as [...] via arch-releng <arch-releng@archlinux.org>, some who's name is spelled incorrectly, with a wrong email address, etc.

For git log and git shortlog -e, this could be solved with a .mailmap file.

Unfortunately .mailmap is currently not supported by GitLab ( https://gitlab.com/gitlab-org/gitlab/-/issues/14909 ), but that shouldn't really prevent implementing this.