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>
diff --git a/builtin/replace.c b/builtin/replace.c
index 6b3c469..748c6ca 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -358,10 +358,10 @@
 
 	/* iterate over new parents */
 	for (i = 1; i < mergetag_data->argc; i++) {
-		unsigned char sha1[20];
-		if (get_sha1(mergetag_data->argv[i], sha1) < 0)
+		struct object_id oid;
+		if (get_sha1(mergetag_data->argv[i], oid.hash) < 0)
 			die(_("Not a valid object name: '%s'"), mergetag_data->argv[i]);
-		if (!hashcmp(tag->tagged->sha1, sha1))
+		if (!oidcmp(&tag->tagged->oid, &oid))
 			return; /* found */
 	}