rebase: move core rebase logic to be a method of the RR class

rebase: move local variable 'extrafn' to the RR class

rebase: move local variable 'currentbookmarks' to the RR class

rebase: make collapsing use explicit logic to decide on the rev to reuse This code: for rev in sortedstate: ... ... newnode = concludenode(repo, rev, p1, rbsrt.external, commitmsg=commitmsg, extrafn=extrafn, editor=editor, keepbranches=rbsrt.keepbranchesf, date=rbsrt.date) uses 'rev' variable in 'concludenode' function invocation. It is not explicitly assigned before, but its value comes as last value or 'rev' in a for loop, e.g. last element in a 'sortedstate'. IMO this a bad style and it also makes it hard to refactor the function, so it is better to explicitly define the value passed to 'concludenode'.

rebase: move new rebase preparation to be a method of the RR class This commit moves logic that prepares the execution of a new rebase operation to be a method of the rebaseruntime class.

rebase: move abort/continue prep to be a method of the RR class This commit moves logic that prepares the execution of abort and continue phases or rebase operation to be a method of the rebaseruntime class.