Make git cat-file support mailmap

Goal

The goal would be to make it possible for git cat-file to output commits and (perhaps) tag objects, where the content of the "author", "committer" and "tagger" fields have been passed through the mailmap mechanism, so that the email addresses and names are updated and not necessary the original email addresses and names.

This would be a big step towards resolving gitlab#14909

Proposed solution

It could be done by adding a new command line option, and/or maybe a config variable, in a similar way as what was done for git log, git shortlog and git show which have:

       --[no-]mailmap, --[no-]use-mailmap
           Use mailmap file to map author and committer names and email addresses to canonical real names and email addresses.
           See git-shortlog(1).

and

https://git-scm.com/docs/git-config#Documentation/git-config.txt-logmailmap