Introduce a new state of branch in status/traverse: removed from remote

[PawelLipski]

This is state is already kind-of supported via git machete slide-out --removed-from-remote. In this command, a branch is considered removed from remote when both conditions are true:

1. git config key branch.BRANCH.merge is set
2. remote branch pointed by this git config key doesn't exist in refs/remotes.

More precisely, slide-out --removed-from-remote only slides out a removed-from-remote branch when it also doesn't have any downstream branches.

Currently, such branches are just considered untracked in every context other than slide-out --removed-from-remote.

This new state would need to be handled in:

• status
• traverse (suggest sliding out, not pushing)
• git{hub,lab} create-{pr,mr} — TBD what to do with such branches.
• IntelliJ plugin.

[PawelLipski]

TBD if it really makes sense — I'd require quite a few changes around traverse and github/gitlab (and also porting the change to the IntelliJ plugin), and there hasn't really been any external request to do that. It just seemed like a relatively easy fix to do alongside #1241 😛