Meta/Reintegrate: force invoking the editor when merging

As "git merge" is clever not to default to --edit behaviour when its standard input and output streams are not connected to the same terminal (i.e. indicating an interactive tty session), we would need to explicitly ask for "--edit" while exporting "EDITOR=:" one-shot, so that commit log cleaner still is triggered. In the longer term, the interactions among pull, fmt-merge-msg, merge and the editor need to be redesigned, but it is a large task, so for now this workaround should suffice.