Move copy_note_for_rewrite + friends from builtin/notes.c to notes-utils.c

This is a pure code movement of the machinery for copying notes to
rewritten objects. This code was located in builtin/notes.c for
historical reasons. In order to make it available to builtin/commit.c
it was declared in builtin.h. This was more of an accident of history
than a concious design, and we now want to make this machinery more
widely available.

Hence, this patch moves the code into the new notes-utils.[hc] files
which are included into libgit.a. Except for adjusting #includes
accordingly, this patch merely moves the relevant functions verbatim
into the new files.

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

builtin/commit.c

@@ -29,6 +29,7 @@
 #include "gpg-interface.h"
 #include "column.h"
 #include "sequencer.h"
+#include "notes-utils.h"
 
 static const char * const builtin_commit_usage[] = {
 	N_("git commit [options] [--] <pathspec>..."),