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/runtime/doc/options.txt b/runtime/doc/options.txt
index 16aa422..a02618a 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -2615,7 +2615,7 @@
 	security reasons.
 
 						*'dip'* *'diffopt'*
-'diffopt' 'dip'		string	(default "filler")
+'diffopt' 'dip'		string	(default "internal,filler")
 			global
 			{not in Vi}
 			{not available when compiled without the |+diff|