Add mailmap support

This patch set adds support for mailmap-based rewriting of user names, emails, and commit signatures. Technically, all mailmap-related procedures can error out, so a GError is required in all those functions, but in practice the implementation appears to be pretty "happy path", particularly when it comes to initializing a mailmap from a repository, which can have quite weird results depending on when you do it (I've experimented a little with gitg on the side and am no wiser than when I started).