notes: implement 'git notes copy --stdin'

This implements a mass-copy command that takes a sequence of lines in
the format

  <from-sha1> SP <to-sha1> [ SP <rest> ] LF

on stdin, and copies each <from-sha1>'s notes to the <to-sha1>.  The
<rest> is ignored.  The intent, of course, is that this can read the
same input that the 'post-rewrite' hook gets.

The copy_note() function is exposed for everyone's and in particular
the next commit's use.

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>

notes.h

@@ -99,6 +99,15 @@ void remove_note(struct notes_tree *t, const unsigned char *object_sha1);
 const unsigned char *get_note(struct notes_tree *t,
 		const unsigned char *object_sha1);
 
+/*
+ * Copy a note from one object to another in the given notes_tree.
+ *
+ * Fails if the to_obj already has a note unless 'force' is true.
+ */
+int copy_note(struct notes_tree *t,
+	      const unsigned char *from_obj, const unsigned char *to_obj,
+	      int force, combine_notes_fn combine_fn);
+
 /*
  * Flags controlling behaviour of for_each_note()
  *