Add Support for Mailmap

GitLab uses Gitaly to read and write git data to the repositories. Gitaly provides RPC access to the git repositories. The RPCs that provide the author and the committer information make use of the git-cat-file command, which doesn’t support mailmap. So, this project aims to add mailmap support in git-cat-file command, make Gitaly to use it and thus enable GitLab to honor the .mailmap file across its various features.