Add pretty format %aN which gives the author name, respecting .mailmap

Add pretty format %aN which gives the author name, respecting .mailmap

The pretty format %an does not respect .mailmap, but gives the exact
author name recorded in the commit.  Sometimes it is more desirable,
however, to look if the email has another name mapped to it in .mailmap.

This commit adds %aN (and %cN for the committer name) to do exactly that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>