Create a .mailmap file as described in git-shortlog(1).  This is used to
map commits that contain different names or email addresses to the same
person.

This file will automatically be used by git-shortlog.  It can also be
used by other commands, such as git-log by providing the --use-mailmap
option.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>