patch 8.0.0300: cannot stop diffing hidden buffers

Problem:    Cannot stop diffing hidden buffers. (Daniel Hahler)
Solution:   When using :diffoff! make the whole list if diffed buffers empty.
            (closes #736)
diff --git a/src/version.c b/src/version.c
index fa51e56..da53eb3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    300,
+/**/
     299,
 /**/
     298,