topics: drop the clean argument from _changetopics() After this commit, None will represent that we want to clean the topic.

topics: rename '--change' flag to '--rev' flag The --change flag was used to read a revset of which topic should be changed. In mercurial we use --rev for this type of flag. Now things will work as follows: `hg topics topicname`: It will set the current topic `hg topics topicname --rev revset`: Change topic of all the revs in the revset to topicname Further patches will try to achieve the following: `hg topics --clear --rev revset`: clear topic from all the revisions in the revset `hg topics --clear`: clear the current topic (Current behavior) `hg topics --rev revset`: show topics on the revisions in the revset

topics: factor out the logic to change topic in a new function It will help to refactor and fix bugs. Moreover we can re-use the logic.