notes: implement 'git notes copy --stdin'

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 <redacted>
Acked-by: Johan Herland <redacted>
Signed-off-by: Junio C Hamano <redacted>