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.

Filelist

@@ -273,6 +273,8 @@ SRC_ALL =	\
 		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 \

runtime/doc/options.txt

@@ -2615,7 +2615,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	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|

src/option.c

@@ -1049,7 +1049,7 @@ static struct vimoption options[] =
 								     |P_NODUP,
 #ifdef FEAT_DIFF
 			    (char_u *)&p_dip, PV_NONE,
-			    {(char_u *)"filler", (char_u *)NULL}
+			    {(char_u *)"internal,filler", (char_u *)NULL}
 #else
 			    (char_u *)NULL, PV_NONE,
 			    {(char_u *)"", (char_u *)NULL}

src/version.c

@@ -794,6 +794,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    363,
 /**/
     362,
 /**/