sequencer: completely revamp the "todo" script parsing

sequencer: completely revamp the "todo" script parsing

When we came up with the "sequencer" idea, we really wanted to have
kind of a plumbing equivalent of the interactive rebase. Hence the
choice of words: the "todo" script, a "pick", etc.

However, when it came time to implement the entire shebang, somehow this
idea got lost and the sequencer was used as working horse for
cherry-pick and revert instead. So as not to interfere with the
interactive rebase, it even uses a separate directory to store its
state.

Furthermore, it also is stupidly strict about the "todo" script it
accepts: while it parses commands in a way that was *designed* to be
similar to the interactive rebase, it then goes on to *error out* if the
commands disagree with the overall action (cherry-pick or revert).

Finally, the sequencer code chose to deviate from the interactive rebase
code insofar that when it comes to writing the file with the remaining
commands, it *reformats* the "todo" script instead of just writing the
part of the parsed script that were not yet processed. This is not only
unnecessary churn, but might well lose information that is valuable to
the user (i.e. comments after the commands).

Let's just bite the bullet and rewrite the entire parser; the code now
becomes not only more elegant: it allows us to go on and teach the
sequencer how to parse *true* "todo" scripts as used by the interactive
rebase itself. In a way, the sequencer is about to grow up to do its
older brother's job. Better.

In particular, we choose to maintain the list of commands in an array
instead of a linked list: this is flexible enough to allow us later on to
even implement rebase -i's reordering of fixup!/squash! commits very
easily (and with a very nice speed bonus, at least on Windows).

While at it, do not stop at the first problem, but list *all* of the
problems. This will help the user when the sequencer will do `rebase
-i`'s work by allowing to address all issues in one go rather than going
back and forth until the todo list is valid.

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