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)
diff --git a/src/version.c b/src/version.c
index 089c134..b705463 100644
--- a/src/version.c
+++ b/src/version.c
@@ -780,6 +780,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    956,
+/**/
     955,
 /**/
     954,