Currently the git segment does not detect when an upstream is removed
(e.g., a PR is merged and the remote branch is deleted).
This change adds functionality to detect when upstream is removed;
if the status reports "branch.upstream" but no "branch.ab", it is gone.
The UI has also been changed to reflect this third state;
it now matches posh-git's behavior, which only shows the Gone icon
if an upstream branch is set but does not report ahead/behind counts.