sequencer: remember the onelines when parsing the todo file

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 offsets and lengths.

As we will have to parse and remember the command-line for `exec` commands
later, we do not call the field "oneline" but rather "arg" (and will reuse
that for exec's command-line).

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