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.

mq: fold the code path to invoke editor into specific logic (qnew) This factoring simplifies the succeeding patch to switch from "ui.edit()" to "getcommiteditor()" for qnew.

cmdutil: enhance "getcommiteditor()" for specific usages in MQ This patch introduces "finishdesc" and "extramsg" arguments into "getcommiteditor()" for specific usages in MQ. "finishdesc" will be used to treat the commit message as "[mq]; patch-file-name" (default MQ commit message), if it is left as empty, instead of aborting commit process. "extramsg" will be used to show the line below: HG: Leave message empty to use default message instead of: HG: Leave message empty to abort commit

tag: use the editor gotten by "getcommiteditor()" instead of "ui.edit()" This patch also enhances "test-tag.t", because "hg tag" hasn't been explicitly tested around editor invocation and "--edit" option.