patch 8.1.0495: :filter only supports some commands

Problem:    :filter only supports some commands.
Solution:   Add :filter support for more commands. (Marcin Szamotulski,
            closes #2856)
diff --git a/src/eval.c b/src/eval.c
index e70bfd8..4a3cf99 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1425,6 +1425,7 @@
     hashitem_T	*hi;
     dictitem_T	*di;
     int		todo;
+    char_u	buf[IOSIZE];
 
     todo = (int)ht->ht_used;
     for (hi = ht->ht_array; todo > 0 && !got_int; ++hi)
@@ -1433,6 +1434,13 @@
 	{
 	    --todo;
 	    di = HI2DI(hi);
+
+	    // apply :filter /pat/ to variable name
+	    vim_strncpy((char_u *) buf, prefix, IOSIZE - 1);
+	    vim_strcat((char_u *) buf, di->di_key, IOSIZE);
+	    if (message_filtered(buf))
+		continue;
+
 	    if (empty || di->di_tv.v_type != VAR_STRING
 					   || di->di_tv.vval.v_string != NULL)
 		list_one_var(di, prefix, first);