The mktree_line() function does not actually look at the "len" parameter
it is passed, and assumes the buffer it receives is NUL-terminated.
Since the caller always passes a strbuf, this will be true. Let's drop
the useless parameter.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>