Add an initial .mailmap

## What does this MR do?

Add an intiail ".mailmap" file to better map users names and email adresses for `git shortlog` and `git check-mailmap` to work better.

## Are there points in the code the reviewer needs to double check?

The reviewer can try `git shortlog -nse` with and without the ".mailmap" to see the difference.

## Why was this MR needed?

It starts to make `git shortlog` and `git check-mailmap` useful.

## What are the relevant issue numbers?

I don't know of any. I searched for "mailmap" and found nothing.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

I can add an entry into the CHANGELOG but this is for developers only so I am not sure it is really needed.

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5583