subtree: initialize "prefix" variable

We parse the "--prefix" command-line option into the "$prefix" shell variable. However, if we do not see such an option, the variable is left with whatever value it had in the environment. We should initialize it to a known value, like we do for other variables. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>