patch 9.1.0351: No test that completing a partial mapping clears 'showcmd'

Problem:  No test that completing a partial mapping clears 'showcmd'.
Solution: Complete partial mappings in Test_showcmd_part_map() instead
          of using :echo.  Adjust some comments (zeertzjq).

closes: #14580

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/getchar.c

@@ -1281,6 +1281,9 @@ del_typebuf(int len, int offset)
 	typebuf.tb_change_cnt = 1;
 }
 
+/*
+ * State for adding bytes to a recording or 'showcmd'.
+ */
 typedef struct
 {
     int		prev_c;