Make gitaly use --use-mailmap option implemented in git cat-file

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 after git#106 (closed)