Move convert-objects to contrib.

Move convert-objects to contrib.

convert-objects was needed to convert from an old-style repository,
which hashed the compressed contents and used a different date format.
Such repositories are presumably no longer common and, if such
conversions are necessary, should be done by writing a frontend for
git-fast-import.

Linus, the original author, is OK with moving it to contrib.

Signed-off-by: Matt Kraai <kraai@ftbfs.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>