Merge branch 'add-mailmap' into 'master'

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

.mailmap

+#
+# This list is used by git-shortlog to make contributions from the
+# same person appearing to be so.
+#
+
+Achilleas Pipinellis <axilleas@axilleas.me> <axilleas@archlinux.gr>
+Achilleas Pipinellis <axilleas@axilleas.me> <axilleas@users.noreply.github.com>
+Dmitriy Zaporozhets <dzaporozhets@gitlab.com> <dmitriy.zaporozhets@gmail.com>
+Dmitriy Zaporozhets <dzaporozhets@gitlab.com> <dzaporozhets@sphereconsultinginc.com>
+Douwe Maan <douwe@gitlab.com> <douwe@selenight.nl>
+Douwe Maan <douwe@gitlab.com> <me@douwe.me>
+Grzegorz Bizon <grzegorz@gitlab.com> <grzegorz.bizon@ntsn.pl>
+Grzegorz Bizon <grzegorz@gitlab.com> <grzesiek.bizon@gmail.com>
+Jacob Vosmaer <jacob@gitlab.com> <contact@jacobvosmaer.nl>
+Jacob Vosmaer <jacob@gitlab.com> Jacob Vosmaer (GitLab) <jacob@gitlab.com>
+Jacob Schatz <jschatz@gitlab.com> <jacobschatz@Jacobs-MacBook-Pro.local>
+Jacob Schatz <jschatz@gitlab.com> <jacobschatz@Jacobs-MBP.fios-router.home>
+Jacob Schatz <jschatz@gitlab.com> <jschatz1@gmail.com>
+James Lopez <james@jameslopez.es> <james@gitlab.com>
+James Lopez <james@jameslopez.es> <james.lopez@vodafone.com>
+Kamil Trzciński <kamil@gitlab.com> <ayufan@ayufan.eu>
+Marin Jankovski <maxlazio@gmail.com> <marin@gitlab.com>
+Phil Hughes <me@iamphill.com> <theephil@gmail.com>
+Rémy Coutable <remy@rymai.me> <remy@gitlab.com>
+Robert Schilling <rschilling@student.tugraz.at> <Razer6@users.noreply.github.com>
+Robert Schilling <rschilling@student.tugraz.at> <schilling.ro@gmail.com>
+Robert Speicher <robert@gitlab.com> <rspeicher@gmail.com>
+Stan Hu <stanhu@gmail.com> <stanhu@alum.mit.edu>
+Stan Hu <stanhu@gmail.com> <stanhu@packetzoom.com>
+Stan Hu <stanhu@gmail.com> <stanhu@users.noreply.github.com>
+Stan Hu <stanhu@gmail.com> stanhu <stanhu@gmail.com>
+Sytse Sijbrandij <sytse@gitlab.com> <sytse+admin@gitlab.com>
+Sytse Sijbrandij <sytse@gitlab.com> <sytse@dosire.com>
+Sytse Sijbrandij <sytse@gitlab.com> <sytses@gmail.com>
+Sytse Sijbrandij <sytse@gitlab.com> dosire <sytse@gitlab.com>