sequencer: don't abbreviate a command if it doesn't have a short form

When the sequencer is requested to abbreviate commands, it will replace
those that do not have a short form (eg. `noop') by a comment mark.
`noop' serves no purpose, except when fast-forwarding (ie. by running
`git rebase').  Removing it will break this command when
`rebase.abbreviateCommands' is set to true.

Teach todo_list_to_strbuf() to check if a command has an actual
short form, and to ignore it if not.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -1564,7 +1564,7 @@ static const char *command_to_string(const enum todo_command command)
 
 static char command_to_char(const enum todo_command command)
 {
-	if (command < TODO_COMMENT && todo_command_info[command].c)
+	if (command < TODO_COMMENT)
 		return todo_command_info[command].c;
 	return comment_line_char;
 }
@@ -4947,6 +4947,8 @@ static void todo_list_to_strbuf(struct repository *r, struct todo_list *todo_lis
 		max = num;
 
 	for (item = todo_list->items, i = 0; i < max; i++, item++) {
+		char cmd;
+
 		/* if the item is not a command write it and continue */
 		if (item->command >= TODO_COMMENT) {
 			strbuf_addf(buf, "%.*s\n", item->arg_len,
@@ -4955,8 +4957,9 @@ static void todo_list_to_strbuf(struct repository *r, struct todo_list *todo_lis
 		}
 
 		/* add command to the buffer */
-		if (flags & TODO_LIST_ABBREVIATE_CMDS)
-			strbuf_addch(buf, command_to_char(item->command));
+		cmd = command_to_char(item->command);
+		if ((flags & TODO_LIST_ABBREVIATE_CMDS) && cmd)
+			strbuf_addch(buf, cmd);
 		else
 			strbuf_addstr(buf, command_to_string(item->command));