patch 9.1.0741: No way to get prompt for input()/confirm()

Problem:  No way to get prompt for input()/confirm()
Solution: add getcmdprompt() function (Shougo Matsushita)
          (Shougo Matsushita)

closes: #15667

Signed-off-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 173b544..7c096a2 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -2097,6 +2097,8 @@
 			ret_string,	    f_getcmdline},
     {"getcmdpos",	0, 0, 0,	    NULL,
 			ret_number,	    f_getcmdpos},
+    {"getcmdprompt",	0, 0, 0,	    NULL,
+			ret_string,	    f_getcmdprompt},
     {"getcmdscreenpos",	0, 0, 0,	    NULL,
 			ret_number,	    f_getcmdscreenpos},
     {"getcmdtype",	0, 0, 0,	    NULL,
@@ -3778,6 +3780,8 @@
     message = tv_get_string_chk(&argvars[0]);
     if (message == NULL)
 	error = TRUE;
+    else
+	set_prompt(message);
     if (argvars[1].v_type != VAR_UNKNOWN)
     {
 	buttons = tv_get_string_buf_chk(&argvars[1], buf);