cat-file: add mailmap support

cat-file: add mailmap support

git-cat-file is used by tools like GitLab to get commit tag contents
that are then displayed to users. This content which has author,
committer or tagger information, could benefit from passing through the
mailmap mechanism before being sent or displayed.

This patch adds --[no-]use-mailmap command line option to the git
cat-file command. It also adds --[no-]mailmap option as an alias to
--[no-]use-mailmap.

This patch also introduces new test cases to test the mailmap mechanism in
git cat-file command.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: John Cai <johncai86@gmail.com>
Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>