clone: use OPT_STRING_LIST for --reference
clone: use OPT_STRING_LIST for --reference

Not only does this save us having to implement a custom
callback, but it handles "--no-reference" in the usual way
(to clear the list).

The generic callback does copy the string, which we don't
technically need, but that should not hurt anything.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git