fetch.c: do not call process_tree() from process_tree().

This function reads a freshly fetched tree object, and schedules
the objects pointed by it for further fetching, so doing
lookup_tree() and process_tree() recursively from there does not
make much sense.  We need to use process() on it to make sure we
fetch it first, and leave the recursive processing to later
stages.

Signed-off-by: Junio C Hamano <junkio@cox.net>