patch 9.1.0677: :keepp does not retain the substitute pattern

Problem:  :keeppatterns does not retain the substitute pattern
          for a :s command
Solution: preserve the last substitute pattern when used with the
          :keeppatterns command modifier (Gregory Anders)

closes: #15497

Signed-off-by: Gregory Anders <greg@gpanders.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    677,
 /**/
     676,
 /**/