bisect: use oid_to_hex_r() instead of memcpy()+oid_to_hex()

bisect: use oid_to_hex_r() instead of memcpy()+oid_to_hex()

Write the hexadecimal object ID directly into the destination buffer
using oid_to_hex_r() instead of writing it into a static buffer first
using oid_to_hex() and then copying it from there using memcpy().
This is shorter, simpler and a bit more efficient.

Reviewed-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>