left over branch configs

I accidentally realised that my `.git/config` is littered with left-over empty branch specs of already removed branches:
```
[branch "teest"]
[branch "fix-1047-can-pull"]
[branch "fix-1037"]
[branch "fix-747-annotated-tags"]
[branch "fix-1074-delete-tag-remote"]
[branch "test-fix-cargo-wix"]
[branch "tree-walk"]
```

Start by inspecting the empty branch sections in .git/config and tracing how gitui handles removed branches or branch configuration. The change is complete when configurations for deleted branches are no longer left behind, with behavior verified against the reported examples.

Vom Indexierungsmodell aus dem Issue-Text verfasst.