provide a copy of the LGPLv2.1

The LGPL seems to require providing a copy of the license when
distributing xdiff, compat/fnmatch, and so on, or altering the license
notices to refer to the GPL intead.  Since we don't want to do the
latter, let's do the former.  It's nice to let people know their
rights anyway.

Inspired-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>