histedit: move `continue` logic into a dedicated function When histedit "continue", there is several complicated logic to apply in order to detect intermediate changeset and concluded pending operation. This changeset extract this logic in a dedicated function to lighten the main one. No alteration to the logic is done.

histedit: rename `tip` to `topmost` I expected `tip` to be repo's tip when it was the rewritten set tip. I rename the variable to the less ambiguous `topmost`.

histedit: factorise node stripping logic Create a function dedicated to stripping a group of node. All existing duplicated code is replaced by call to this function. This new function take care of stripping known and relevant node only.

histedit: extract bookmark logic in a dedicated function This lighten the main function and will help to see future changes to this bookmark logic.