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.