t: add an expensive test to verify that 4GB+ blobs can be staged/read

I should have added this test as soon as
git-for-windows#6012 was opened.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/t3706-add-4gb-file.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+#
+# Copyright (c) 2026 Johannes Schindelin
+#
+
+test_description='add 4GB+ objects'
+
+. ./test-lib.sh
+
+size_4gb=4294967296
+
+test_expect_success EXPENSIVE 'set up a 4GB file' '
+	test_atexit "rm -f large" &&
+	# genrandom takes only an unsigned long...
+	test-tool genrandom 123 $(($size-1)) >large &&
+	printf 1 >>large
+'
+
+test_expect_success 'add 4GB file' '
+	git add large &&
+	git cat-file -s :large >size-staged &&
+	test $size_4gb = $(cat size-staged)
+'
+test_expect_success 'read 4GB loose object' '
+	git -P show :large >read &&
+	test_file_size read >size-read &&
+	test $size_4gb = $(cat size-read)
+'
+
+test_done
+