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>

entry.c

@@ -266,7 +266,7 @@ static int write_entry(struct cache_entry *ce,
 
 	if (ce_mode_s_ifmt == S_IFREG) {
 		struct stream_filter *filter = get_stream_filter(ce->name,
-								 ce->oid.hash);
+								 &ce->oid);
 		if (filter &&
 		    !streaming_write_entry(ce, path, filter,
 					   state, to_tempfile,