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.

Reintegrate

@@ -100,7 +100,7 @@ no)
 			mb=$(git merge-base "$tip" "$save")
 			test "$mb" = "$tip" && continue
 
-			EDITOR=: git merge $accept_rerere "$branch" ||
+			EDITOR=: git merge $accept_rerere --edit "$branch" ||
 			accept_rerere ||
 			exit