patch 9.0.1714: getcompletion() "cmdline" fails after :autocmd

Problem:  getcompletion() "cmdline" fails after :autocmd
Solution: Use set_cmd_context() instead of set_one_cmd_context().

closes: #12804

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/src/cmdexpand.c b/src/cmdexpand.c
index 0969106..61532b9 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -4017,9 +4017,10 @@
     ExpandInit(&xpc);
     if (STRCMP(type, "cmdline") == 0)
     {
-	set_one_cmd_context(&xpc, pat);
+	int cmdline_len = (int)STRLEN(pat);
+	set_cmd_context(&xpc, pat, cmdline_len, cmdline_len, FALSE);
 	xpc.xp_pattern_len = (int)STRLEN(xpc.xp_pattern);
-	xpc.xp_col = (int)STRLEN(pat);
+	xpc.xp_col = cmdline_len;
     }
     else
     {