mq: smarter handling of plain headers 6333412245ec and 5ccced6eab0b fixed issue4453 with a simple insertplainheader function that fixed the regression but didn't make the implementation more stable. Now we introduce plain header handling similar to how we handle hg patches. The whole header is scanned for fields to update while determining the best position for inserting the field if it is missing. It also makes sure there is an empty line between headers and body.

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.