patch 8.2.1978: making a mapping work in all modes is complicated

Problem:    Making a mapping work in all modes is complicated.
Solution:   Add the <Cmd> special key. (Yegappan Lakshmanan, closes #7282,
            closes 4784, based on patch by Bjorn Linse)
diff --git a/src/ex_getln.c b/src/ex_getln.c
index 1325f77..a00d6de 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -1711,6 +1711,10 @@
 	    c = safe_vgetc();
 	while (c == K_IGNORE || c == K_NOP);
 
+	if (c == K_COMMAND
+		   && do_cmdline(NULL, getcmdkeycmd, NULL, DOCMD_NOWAIT) == OK)
+	    goto cmdline_changed;
+
 	if (KeyTyped)
 	{
 	    some_key_typed = TRUE;