rebase: introduce support for automatically rebasing orphan changes _destautorebase(SRC) is based on the _destrestack(SRC) revset from fbamend. The supporting _possibledestination function is extracted from evolve, with minor cleanups. We've considered some alternatives here: * This change, but with --auto as the flag name. We're hedging our bets on this a little in this change so that if this ends up being the wrong direction we haven't burned the valauble --auto name on rebase. * --destination auto: I've got reservations about the discoverability of this, and we don't currently have a good story for a revset alias of sorts that changes behavior depending on the context in which it's used. * A "rebase presets" feature, where we could use the currently-an-error positional argument space for the rebase command to define presets, so that users could define a 'linearize' preset that specifies --revision='orphan()-obsolete()' and --dest=_destautoorphanrebase(SRC). Personally, I find the third option somewhat appealing, but am hesitant to "spend" the functionality space of positional arguments to the rebase command. We should revisit the way we expose this functionality sometime in the 4.7 cycle once we've had a chance to vet the implementation of the functionality. Differential Revision: https://phab.mercurial-scm.org/D2668