mq: use the editor gotten by "getcommiteditor()" instead of "ui.edit()" (qnew) This patch also replaces "editor = False" by "editor = cmdutil.getcommiteditor()", because: - the latter allows to hook commit message determination easily, even in the case without "--edit" - the latter can avoid regression (or additional care) around saving "last-message.txt", even if MQ's "newcommit()" changes its implementation logic from "localrepository.commit" to "localrepository.commitctx" with "memctx" in the future to save commit message into "last-messge.txt" with "memctx", "editor" should be valid function.