rebase: remove complex unhiding code This is similar to Martin von Zweigbergk's previous patch [1]. Previous patches are adding more `.unfiltered()` to the rebase code. So I wonder: are we playing whack-a-mole regarding on `unfiltered()` in rebase? Thinking about it, I believe most of the rebase code *should* just use an unfiltered repo. The only exception is before we figuring out a `rebasestate`. This patch makes it so. See added comment in code for why that's more reasonable. This would make the code base cleaner (not mangling the `repo` object), faster (no need to invalidate caches), simpler (less LOC), less error-prone (no need to think about what to unhide, ex. should we unhide wdir p2? how about destinations?), and future proof (other code may change visibility in an unexpected way, ex. directaccess may make the destination only visible when it's in "--dest" revset tree). [1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/094277.html Differential Revision: https://phab.mercurial-scm.org/D644