log: grep author/committer using mailmap

log: grep author/committer using mailmap

Currently you can use mailmap to display log authors and committers
but you can't use the mailmap to find commits with mapped values.

This commit allows you to run:

    git log --use-mailmap --author mapped_name_or_email
    git log --use-mailmap --committer mapped_name_or_email

Of course it only works if the --use-mailmap option is used.

The new name and email are copied only when necessary.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>