tree-diff: catch integer overflow in combine_diff_path allocation

A combine_diff_path struct has two "flex" members allocated
alongside the struct: a string to hold the pathname, and an
array of parent pointers. We use an "int" to compute this,
meaning we may easily overflow it if the pathname is
extremely long.

We can fix this by using size_t, and checking for overflow
with the st_add helper.

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