sequencer (rebase -i): implement the 'noop' command

The 'noop' command is probably the most boring of all rebase -i commands
to support in the sequencer.

Which makes it an excellent candidate for this first stab to add support
for rebase -i's commands to the sequencer.

For the moment, let's also treat empty lines and commented-out lines as
'noop'; We will refine that handling later in this patch series.

To make it easier to identify "classes" of todo_commands (such as:
determine whether a command is pick-like, i.e. handles a single commit),
let's enforce a certain order of said commands.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -607,14 +607,23 @@ static int allow_empty(struct replay_opts *opts, struct commit *commit)
 		return 1;
 }
 
+/*
+ * Note that ordering matters in this enum. Not only must it match the mapping
+ * below, it is also divided into several sections that matter.  When adding
+ * new commands, make sure you add it in the right section.
+ */
 enum todo_command {
+	/* commands that handle commits */
 	TODO_PICK = 0,
-	TODO_REVERT
+	TODO_REVERT,
+	/* commands that do nothing but are counted for reporting progress */
+	TODO_NOOP
 };
 
 static const char *todo_command_strings[] = {
 	"pick",
-	"revert"
+	"revert",
+	"noop"
 };
 
 static const char *command_to_string(const enum todo_command command)
@@ -624,6 +633,10 @@ static const char *command_to_string(const enum todo_command command)
 	die("Unknown command: %d", command);
 }
 
+static int is_noop(const enum todo_command command)
+{
+	return TODO_NOOP <= (size_t)command;
+}
 
 static int do_pick_commit(enum todo_command command, struct commit *commit,
 		struct replay_opts *opts)
@@ -879,6 +892,14 @@ static int parse_insn_line(struct todo_item *item, const char *bol, char *eol)
 	/* left-trim */
 	bol += strspn(bol, " \t");
 
+	if (bol == eol || *bol == '\r' || *bol == comment_line_char) {
+		item->command = TODO_NOOP;
+		item->commit = NULL;
+		item->arg = bol;
+		item->arg_len = eol - bol;
+		return 0;
+	}
+
 	for (i = 0; i < ARRAY_SIZE(todo_command_strings); i++)
 		if (skip_prefix(bol, todo_command_strings[i], &bol)) {
 			item->command = i;
@@ -887,6 +908,13 @@ static int parse_insn_line(struct todo_item *item, const char *bol, char *eol)
 	if (i >= ARRAY_SIZE(todo_command_strings))
 		return -1;
 
+	if (item->command == TODO_NOOP) {
+		item->commit = NULL;
+		item->arg = bol;
+		item->arg_len = eol - bol;
+		return 0;
+	}
+
 	/* Eat up extra spaces/ tabs before object name */
 	padding = strspn(bol, " \t");
 	if (!padding)
@@ -1289,7 +1317,12 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 		struct todo_item *item = todo_list->items + todo_list->current;
 		if (save_todo(todo_list, opts))
 			return -1;
-		res = do_pick_commit(item->command, item->commit, opts);
+		if (item->command <= TODO_REVERT)
+			res = do_pick_commit(item->command, item->commit,
+					opts);
+		else if (!is_noop(item->command))
+			return error(_("unknown command %d"), item->command);
+
 		todo_list->current++;
 		if (res)
 			return res;