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/ops.c b/src/ops.c
index 5abe151..cc25683 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -3490,7 +3490,7 @@
 		    AppendToRedobuffLit(cap->searchbuf, -1);
 		AppendToRedobuff(NL_STR);
 	    }
-	    else if (cap->cmdchar == ':')
+	    else if (cap->cmdchar == ':' || cap->cmdchar == K_COMMAND)
 	    {
 		// do_cmdline() has stored the first typed line in
 		// "repeat_cmdline".  When several lines are typed repeating