largefiles: drop the override for 'fetch' The fetch extension has been calling cmdutil.bailifchanged() since a014fdc97154, so this is redundant. Add test coverage to prevent regression. It doesn't look like there is any testing for fetch with largefiles.

rebase: ignore negative state when updating back to original wc parent The state mapping also contains some magic negative values (detached parent, ignored revision). Blindly reading the state thus lead to unfortunate usage of the negative value as an update destination. We now filter them out. We do a minor alteration of the test to catch this.