sequencer: make rearrange_squash() a bit more obvious

There are some commands that have to be skipped from rearranging by virtue
of not handling any commits.

However, the logic was not quite obvious: it skipped commands based on
their position in the enum todo_command.

Instead, let's make it explicit that we skip all commands that do not
handle any commit. With one exception: the `drop` command, because it,
well, drops the commit and is therefore not eligible to rearranging.

Note: this is a bit academic at the moment because the only time we call
`rearrange_squash()` is directly after generating the todo list, when we
have nothing but `pick` commands anyway.

However, the upcoming `merge` command *will* want to be handled by that
function, and it *can* handle commits.

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

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