Actually, there's no need for those XXX to exists.  add_delta calls
got_delta_open that already takes on off_t and does the right thing,
so we can easily fix also this size_t implicit cast:

diff /home/op/w/got
commit - ad4cc36168576274131539e87b9007ef9b3c3725
path + /home/op/w/got
blob - 36efe8c30ade7284694bc600951d79771cc42662
file + lib/pack.c
--- lib/pack.c
+++ lib/pack.c
@@ -1033,7 +1033,7 @@ add_delta(struct got_delta_chain *deltas, off_t delta_
 
 static const struct got_error *
 add_delta(struct got_delta_chain *deltas, off_t delta_offset, size_t tslen,
-    int delta_type, size_t delta_size, size_t delta_data_offset)
+    int delta_type, size_t delta_size, off_t delta_data_offset)
 {
 	struct got_delta *delta;
 
@@ -1076,7 +1076,7 @@ resolve_offset_delta(struct got_delta_chain *deltas,
 	}
 
 	err = add_delta(deltas, delta_offset, tslen, delta_type, delta_size,
-	    delta_data_offset);	/* XXX: off_t vs size_t! */
+	    delta_data_offset);
 	if (err)
 		return err;
 
@@ -1151,7 +1151,7 @@ resolve_ref_delta(struct got_delta_chain *deltas, stru
 	}
 
 	err = add_delta(deltas, delta_offset, tslen, delta_type, delta_size,
-	    delta_data_offset);	/* XXX: off_t vs size_t */
+	    delta_data_offset);
 	if (err)
 		return err;