sha1_file: use hex_to_bytes()

sha1_file: use hex_to_bytes()

The path of a loose object contains its hash value encoded into two
substrings of 2 and 38 hexadecimal digits separated by a slash.  The
first part is handed to for_each_file_in_obj_subdir() in decoded form as
subdir_nr.  The current code builds a full hexadecimal representation of
the hash in a temporary buffer, then uses get_oid_hex() to decode it.

Avoid the intermediate step by taking subdir_nr as-is and using
hex_to_bytes() directly on the second substring.  That's shorter and
easier.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>