Add support for mailmap configuration
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=14909) </details> <!--IssueSummary end--> Following the spirit of improving GitLab for large open source project; GitLab could add support for `.mailmap` configuration. The idea would be that this way any open source contributor that is using more than 1 email address in git can claim his/her open source contributions just by adding this standard git configuration file to the project. This is something that happens mostly when the developer is using at least 2 hosts where each one has a different git configuration (personal/professional). Probably once this file is added/touched gitlab should trigger some kind of background processing to assign the right contributions to the right user as those contributions would be changing owner. Documentation about the feature: https://git-scm.com/docs/git-shortlog#_mapping_authors For reference of how this impacts open source contributors you can listen to the bikeshed podcast: http://bikeshed.fm/35
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