Study if/how could GitLab's contributor graph use git shortlog

Goal

The goal is to find if a call to git shortlog could provide enough information to display the contributor graph. There might be information needed by the graph that is not easily provided by git shortlog yet, and in this case it could be interesting to find if new options could be added to git shortlog so that it would provide the required information. (Developing those new options is not part of this issue, we would create another issue for that.)

The nice thing is that git shortlog already has mailmap support.

This would help resolve gitlab#14909 and could make the contributor graph significantly faster.

Note that this issue could perhaps have been created in the GitLab or Gitaly project. I think that it's quite relevant to Git too though.