tree-walk: use size_t consistently

tree-walk: use size_t consistently

We store and manipulate the cumulative traverse_info.pathlen as an
"int", which can overflow when we are fed ridiculously long pathnames
(e.g., ones at the edge of 2GB or 4GB, even if the individual tree entry
names are smaller than that). The results can be confusing, though
after some prodding I was not able to use this integer overflow to cause
an under-allocated buffer.

Let's consistently use size_t to generate and store these, and make
sure our addition doesn't overflow.

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