Convert struct object to object_id

struct object is one of the major data structures dealing with object
IDs.  Convert it to use struct object_id instead of an unsigned char
array.  Convert get_object_hash to refer to the new member as well.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>

builtin/fmt-merge-msg.c

@@ -378,7 +378,7 @@ static void shortlog(const char *name,
 
 		if (!sb.len)
 			string_list_append(&subjects,
-					   sha1_to_hex(commit->object.sha1));
+					   oid_to_hex(&commit->object.oid));
 		else
 			string_list_append(&subjects, strbuf_detach(&sb, NULL));
 	}