feature request: allow use of .mailmap

[rgommers]

A .mailmap file is used to map commit author info where committers use multiple names or email addresses to a single unique name/email. It is usually located in the root of a repo. See e.g. https://stackoverflow.com/questions/13777171/configuring-git-log-to-use-mailmap-by-default

I cannot find any mention of the feature in this repo; GitPyhon seems to ignore the file. It would be quite useful to allow using that file. Maybe replacing the author name and email in raw, or adding a second row-mailmapped column for example.

[Byron]

As this library is in maintenance mode, essentially everything has to be contributed. Most of the time, this is only bugfixes. However, it should be possible to workaround these issues by using git directly, and leverage the git program in these cases.

[rgommers]

Thanks for the feedback @Byron. The easiest workaround seems to be to read in the .mailmap file separately into a dictionary, and fix the author names with that after extracting them from the 'raw' commit data returned by Repo.