branching: open the mercurial 4.2 compatibility branch The freeze as started.

readme: add an entry about the unified message

touch: use precheck to validate revision This gives us standard check and standard error message.

prune: use precheck to validate revision This gives us standard check and standard error message.

uncommit: use precheck to validate revision This gives us standard check and standard error message.

amend: use precheck to validate revision This gives us standard check and standard error message.

split: use precheck to validate revision This gives us standard check and standard error message.

precheck: handle rewrite attempt of nullrev This is a common breaking case, so we handle and test it in the generic code.

precheck: update the orphan message to something me generic This will help to reuse it for other commands.

precheck: point to the instability doc when aborting This will help user to understand what is going on.

evolve: document the various instability/troubles We need something to point people at.

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.