TODO: sequencer: completely revamp the "todo" script parsing

TODO: remove the check that disallows TODO_PICK with REPLAY_REVERT
and TODO_REVERT with REPLAY_CHERRY_PICK in the previous commit.

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 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.

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

Author: dscho