prepare 2.8 release

grab: update the alias for 2.8 compatibility 2.8 does not update on rebase anymore. We need to update by hand. this makes this alias more fragile.

gdown/gup: add next/previous alias Up and down refer to the customary order in log output rather than concepts; as such, it may be confusing to the user. I for one found them suprising -- they just didn't fit with my mental model. Previous and next, however, imply a clear parent/child relationship. We attroduce the new name as aliases for now.

evolve: allow updating to the successor if the parent is obsolete Although 'hg update' handles this as well, it will also update to tipmost descendent of the successor, if any.

evolve: abort on unknown divergent changeset base ...rather than noisily dying with a KeyError.

test: prepare test for core changeset 6cd9b8428104 Changeset 6cd9b8428104 in Mercurial core (2.8) make rebase preserve more extra. this mean hash changes. We make the test resilient to that.

test: prepare test for core changeset 9c78ed396075 New changeset in mercurial 2.8 (9c78ed396075) change rebase behavior to preverse the original working directory. This break an handfull of test so we preventively update to tip after rebase.