Fixed issue where the thread name cleared when setting it to anything with the same prefix. The bug was with the `firstLine` function in `onChangeName`.
`firstLine` in `onChangeName` was introduced in D863 and D862 to make it impossible to set a thread title with newlines, this fix still ensures that.
And for good measure, this also helps resolve the issue in D3980, where the `ThreadSettingsGeneralTab` button doesn't stay disabled if there are no queued changes.