subrepo: introduce storeclean method Currently this method is unused and it is not implemented for any specific subrepo type (it always returns False). It receives a remote repository path because a repository may have a clean store versus a given repository but not versus another.

subrepo: introduce storeclean helper functions These helper functions are currently unused but will be used to implement the cleanstore method that will be introduced later.

dispatch: exit with status 1 for an InterventionRequired exception (bc)

histedit: switch from util.Abort to util.InterventionRequired where appropriate (bc)