finish_copy_notes_for_rewrite(): Let caller provide commit message

finish_copy_notes_for_rewrite(): Let caller provide commit message

When copying notes for a rewritten object, the resulting notes commit
would have the following hardcoded commit message:

  Notes added by 'git notes copy'

This is obviously bogus when the notes rewriting is performed by
'git commit --amend'.

Therefore, let the caller specify an appropriate notes commit message
instead of hardcoding it. The above message is used for 'git notes copy',
but when calling finish_copy_notes_for_rewrite() from builtin/commit.c,
we use the following message instead:

  Notes added by 'git commit --amend'

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