patch 9.1.0808: Terminal scrollback doesn't shrink when decreasing 'termwinscroll'

Problem:  Terminal scrollback doesn't shrink when reducing
          'termwinscroll'
Solution: Check if option value was decreased (Milly).

closes: #15904

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/optiondefs.h b/src/optiondefs.h
index 42e62e7..ccc527e 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -2563,7 +2563,7 @@
 			    SCTX_INIT},
     {"termwinscroll", "twsl", P_NUM|P_VI_DEF|P_VIM|P_RBUF,
 #ifdef FEAT_TERMINAL
-			    (char_u *)&p_twsl, PV_TWSL, NULL, NULL,
+			    (char_u *)&p_twsl, PV_TWSL, did_set_termwinscroll, NULL,
 			    {(char_u *)10000L, (char_u *)10000L}
 #else
 			    (char_u *)NULL, PV_NONE, NULL, NULL,