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>