patch 9.1.0046: :drop does not re-use empty buffer

Problem:  :drop does not re-use empty buffer
          (Rocco Mao)
Solution: Make :drop re-use an empty buffer
          (Rocco Mao)

fixes: #13851
closes: #13881

Signed-off-by: Rocco Mao <dapeng.mao@qq.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/arglist.c b/src/arglist.c
index d28b31d..292af30 100644
--- a/src/arglist.c
+++ b/src/arglist.c
@@ -502,7 +502,7 @@
     void
 set_arglist(char_u *str)
 {
-    do_arglist(str, AL_SET, 0, FALSE);
+    do_arglist(str, AL_SET, 0, TRUE);
 }
 
 /*