sequencer: get rid of the subcommand field

sequencer: get rid of the subcommand field

The subcommands are used exactly once, at the very beginning of
sequencer_pick_revisions(), to determine what to do. This is an
unnecessary level of indirection: we can simply call the correct
function to begin with. So let's do that.

While at it, ensure that the subcommands return an error code so that
they do not have to die() all over the place (bad practice for library
functions...).

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