/usr/share/debhelper/dh-kpatches/register-patch is in kernel-patch-scripts 0.99.36+nmu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | #!/bin/sh
# This scripts documents the @PATCHNAME@ kernel patch into the
# kernel-image package, as being applied to the kernel.
docdir=${IMAGE_TOP}/usr/share/doc/kernel-image-${version}
mkdir -p ${docdir}
echo @PATCHNAME@ '(@PATCHID@), from package @PKGNAME@, version @PKGVERS@' >> ${docdir}/applied-patches
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