convert: convert to struct object_id

convert: convert to struct object_id

Convert convert.c to struct object_id.  Add a use of the_hash_algo to
replace hard-coded constants and change a strbuf_add to a strbuf_addstr
to avoid another hard-coded constant.

Note that a strict conversion using the hexsz constant would cause
problems in the future if the internal and user-visible hash algorithms
differed, as anticipated by the hash function transition plan.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>