fast-import: Add support for importing commit notes

fast-import: Add support for importing commit notes

Introduce a 'notemodify' subcommand of the 'commit' command. This subcommand
is similar to 'filemodify', except that no mode is supplied (all notes have
mode 0644), and the path is set to the hex SHA1 of the given "comittish".

This enables fast import of note objects along with their associated commits,
since the notes can now be named using the mark references of their
corresponding commits.

The patch also includes a test case of the added functionality.

Signed-off-by: Johan Herland <johan@herland.net>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>