Clean-up read-tree error condition.

Clean-up read-tree error condition.

This is a follow-up to f34f2b0b; list_tree() function is where it
first notices that the command line fed too many trees for us to
handle, so move the error exit message to there, and raise the
MAX_TREES to 8 (not that it matters very much in practice).

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>