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>
diff --git a/builtin/commit.c b/builtin/commit.c
index d2f30d9..f8df8ca 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1591,7 +1591,7 @@
 		if (cfg) {
 			/* we are amending, so current_head is not NULL */
 			copy_note_for_rewrite(cfg, current_head->object.sha1, sha1);
-			finish_copy_notes_for_rewrite(cfg);
+			finish_copy_notes_for_rewrite(cfg, "Notes added by 'git commit --amend'");
 		}
 		run_rewrite_hook(current_head->object.sha1, sha1);
 	}