patch 8.1.0363: internal diff isn't used by default as advertised

Problem:    Internal diff isn't used by default as advertised.
Solution:   Add "internal" to the default value of 'diffopt'.
            Also add couple of files missing from the distribution.
diff --git a/Filelist b/Filelist
index 343a2bd..59415d3 100644
--- a/Filelist
+++ b/Filelist
@@ -273,6 +273,8 @@
 		src/libvterm/t/92lp1640917.test \
 		src/libvterm/t/harness.c \
 		src/libvterm/t/run-test.pl \
+		src/xdiff/COPYING \
+		src/xdiff/README.txt \
 		src/xdiff/xdiff.h \
 		src/xdiff/xdiffi.c \
 		src/xdiff/xdiffi.h \