fix: Fix crash when current head no longer exists

[dotdash]

When e.g. issuing a jj new command in a different window that causes the current head to disappear (because it was empty), we currently crash when trying to find the latest version of that head. Instead, we should just switch to the @ revision in that case.

[peso]

I think self.set_head(..) will set current head, refresh files, and refresh diff. Why not call self.set_head in both cases? This would lead to a simpler solution (warning! not tested with compiler)

let next_head = commander.get_head_latest(&self.head)
  .or_else(|_| commander.get_current_head())?;
self.set_head(commander, next_head);

[dotdash]

I think there was something that I thought made this necessary, but neither did I document it, nor can I recall it, and @Edgpaez's fix seems to work just fine. And we likely can't merge this PR anyway due to having no reviewers 🤷🏻

[Edgpaez]

This also crashes when doing a fetch that removes the current head (for instance because you were in a branch that is merged).

I took a look and I think this is caused by #181
The problem is that jj log --no-graph -r luwt fails if luwt does not exists. However, after #181 we do jj log --no-graph -r 'change_id(luwt)' that simply returns empty if luwt does not exists.

That fixes the crash but doesn't update the UI to show the new change. For that, we also need to do self.set_head as proposed by @peso

I implemented this in #188 that fixes this in both log_tab and files_tab 👍