git +edit setting for "Default branch" marked as "(Optional)" is not optional

[Barry Price]

When editing a private git repository, omitting a value for the "Default branch" setting (marked in the UI as "(Optional)" results in the error "Required input is missing."

If the field isn't always optional, it shouldn't be marked as so.

Assuming it is indeed optional in some cases, ideally the form would dynamically figure out whether that's the case and mark it accordingly.

Otherwise the "(Optional)" text should either be removed or clarified.

[Ines Almeida (ines-almeida)]

Indeed in GitRepositoryEditView.setUpWidgets() we seem to make it `required = True` if it already exists:

```
if self.context.default_branch:
     self.widgets["default_branch"].context.required = True
```

I assume then when editing (not creating) this always exists already, so it doesn't make sense to have the '(Optional)' there when editing.