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