patch 8.1.0956: using context:0 in 'diffopt' does not work well

Problem:    Using context:0 in 'diffopt' does not work well.
Solution:   Make zero context do the same as one line context. (closes #4005)

src/version.c

@@ -779,6 +779,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    956,
 /**/
     955,
 /**/