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>

streaming.h

@@ -12,4 +12,6 @@ extern struct git_istream *open_istream(const unsigned char *, enum object_type
 extern int close_istream(struct git_istream *);
 extern ssize_t read_istream(struct git_istream *, char *, size_t);
 
+extern int stream_blob_to_fd(int fd, const unsigned char *, struct stream_filter *, int can_seek);
+
 #endif /* STREAMING_H */