patch 8.2.1901: variable completion does not work in command line window

Problem:    Variable completion does not work in command line window.
Solution:   Use the "prevwin". (closes #7198)
diff --git a/src/evalvars.c b/src/evalvars.c
index 1f418ae..a5db6e9 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1981,7 +1981,13 @@
     }
 
     // b: variables
-    ht = &curbuf->b_vars->dv_hashtab;
+    ht =
+#ifdef FEAT_CMDWIN
+	// In cmdwin, the alternative buffer should be used.
+	(cmdwin_type != 0 && get_cmdline_type() == NUL) ?
+	&prevwin->w_buffer->b_vars->dv_hashtab :
+#endif
+	&curbuf->b_vars->dv_hashtab;
     if (bdone < ht->ht_used)
     {
 	if (bdone++ == 0)
@@ -1994,7 +2000,13 @@
     }
 
     // w: variables
-    ht = &curwin->w_vars->dv_hashtab;
+    ht =
+#ifdef FEAT_CMDWIN
+	// In cmdwin, the alternative window should be used.
+	(cmdwin_type != 0 && get_cmdline_type() == NUL) ?
+	&prevwin->w_vars->dv_hashtab :
+#endif
+	&curwin->w_vars->dv_hashtab;
     if (wdone < ht->ht_used)
     {
 	if (wdone++ == 0)