sequencer: remember the onelines when parsing the todo file

The `git-rebase-todo` file contains a list of commands. Most of those
commands have the form

	<verb> <sha1> <oneline>

The <oneline> is displayed primarily for the user's convenience, as
rebase -i really interprets only the <verb> <sha1> part. However, there
are *some* places in interactive rebase where the <oneline> is used to
display messages, e.g. for reporting at which commit we stopped.

So let's just remember it when parsing the todo file; we keep a copy of
the entire todo file anyway (to write out the new `done` and
`git-rebase-todo` file just before processing each command), so all we
need to do is remember the begin and end offsets.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

sequencer.c

@@ -713,6 +713,8 @@ static int read_and_refresh_cache(struct replay_opts *opts)
 struct todo_item {
 	enum todo_command command;
 	struct commit *commit;
+	const char *arg;
+	int arg_len;
 	size_t offset_in_buf;
 };
 
@@ -764,6 +766,9 @@ static int parse_insn_line(struct todo_item *item, const char *bol, char *eol)
 	status = get_sha1(bol, commit_sha1);
 	*end_of_object_name = saved;
 
+	item->arg = end_of_object_name + strspn(end_of_object_name, " \t");
+	item->arg_len = (int)(eol - item->arg);
+
 	if (status < 0)
 		return -1;
 
@@ -890,6 +895,8 @@ static int walk_revs_populate_todo(struct todo_list *todo_list,
 
 		item->command = command;
 		item->commit = commit;
+		item->arg = NULL;
+		item->arg_len = 0;
 		item->offset_in_buf = todo_list->buf.len;
 		subject_len = find_commit_subject(commit_buffer, &subject);
 		strbuf_addf(&todo_list->buf, "%s %s %.*s\n",