precheck: also check for unstability creation

prefetc: point at the offending changeset when aborting We do our best to display comptact but usable information.

precheck: point to 'phase' help when aborting some of the error message already does that. It seems a good idea.

precheck: use 'changesets' in the message instead of 'revisions' This match the message `hg commit --amend` issue in core.

rewriteutil: add a precheck function to validate rewrite beforehand For now this only checks for changeset mutability and this is only used by 'checkfold'. We'll expand the checking and usage as we go.

test: remove reference to the graphlog extension The feature is in core for ages.

test: use common setup for test-split.t This make things cleaner.

split: compute revision to split after locking This would avoid strange behavior from race condition.

split: basic code cleanup We remove a useless extra variable since another one already hold the same value.

commands: rewrite the 'evocommands' module to 'cmdrewrite' The remaining commands should get their own module, so we rename the module for clarity. We update the module comments to better fits its purpose.