unpack-trees: reduce malloc in cache-tree walk

This is a micro optimization that probably only shines on repos with
deep directory structure. Instead of allocating and freeing a new
cache_entry in every iteration, we reuse the last one and only update
the parts that are new each iteration.

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

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