streaming: make streaming-write-entry to be more reusable

streaming: make streaming-write-entry to be more reusable

The static function in entry.c takes a cache entry and streams its blob
contents to a file in the working tree.  Refactor the logic to a new API
function stream_blob_to_fd() that takes an object name and an open file
descriptor, so that it can be reused by other callers.

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