Implement helper functions to load the rewriting config, and to
actually copy the notes.  Also document the config.

Secondly, also implement an undocumented --for-rewrite=<cmd> option to
'git notes copy' which is used like --stdin, but also puts the
configuration for <cmd> into effect.  It will be needed to support the
copying in git-rebase.
Signed-off-by: NThomas Rast <trast@student.ethz.ch>
Acked-by: NJohan Herland <johan@herland.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>