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: Thomas Rast <trast@student.ethz.ch> Acked-by: Johan Herland <johan@herland.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>