http-push: use hex_to_bytes()

http-push: use hex_to_bytes()

The path of a loose object contains its hash value encoded into two
substrings of hexadecimal digits, separated by a slash.  The current
code copies the pieces into a temporary buffer to get rid of the slash
and then uses get_oid_hex() to decode the hash value.

Avoid the copy by using hex_to_bytes() directly on the substrings.
That's shorter and easier.

While at it correct the length of the second substring in a comment.

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