tree.c: update read_tree_recursive callback to pass strbuf as base

tree.c: update read_tree_recursive callback to pass strbuf as base

This allows the callback to use 'base' as a temporary buffer to
quickly assemble full path "without" extra allocation. The callback
has to restore it afterwards of course.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>