Allow to choose email used by merge commit

Description

When having multiple email addresses setup in gitlab, it is not possible to decide which one will be used for the merge commit.

It is useful for example when working on a company project where a specific email is used and at the same time continue to use personal emails to receive notifications and to contribute to other projects.

Currently the only solution is to change the primary email of the user…

Proposal

Simplest solution is to give the choice on each merge request, in the same way that one can modify the commit message, one could choose the commit email.

A good solution would be to choose per group which email to use, in the same way it is done on github I think.

Best solution would be to allow for choosing email per-project (on top of per-group).