pretty: add "%aL"|"%al|%cL|%cl" option to output local-part of email addresses

pretty: add "%aL"|"%al|%cL|%cl" option to output local-part of email addresses

In many projects the number of contributors is low enough that users know
each other and the full email address doesn't need to be displayed.
Displaying only the author's username saves a lot of columns on the screen.
For example displaying "prarit" instead of "prarit@redhat.com" saves 11
columns.

Add a "%aL"|"%al|%cL|%cl" option that output the local-part of an email
address.

Also add tests for "%ae","%an", "%ce", and "%cn".

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>