midx: don't update finalized hash

Ensure that we don't update the finalized commit graph hash; clear it
instead.

Author: ethomson

src/libgit2/midx.c

@@ -660,9 +660,11 @@ static int midx_write_hash(const char *buf, size_t size, void *data)
 	struct midx_write_hash_context *ctx = (struct midx_write_hash_context *)data;
 	int error;
 
-	error = git_hash_update(ctx->ctx, buf, size);
-	if (error < 0)
-		return error;
+	if (ctx->ctx) {
+		error = git_hash_update(ctx->ctx, buf, size);
+		if (error < 0)
+			return error;
+	}
 
 	return ctx->write_cb(buf, size, ctx->cb_data);
 }
@@ -863,6 +865,9 @@ static int midx_write(
 	error = git_hash_final(checksum, &ctx);
 	if (error < 0)
 		goto cleanup;
+
+	hash_cb_data.ctx = NULL;
+
 	error = write_cb((char *)checksum, checksum_size, cb_data);
 	if (error < 0)
 		goto cleanup;