Interactive 3 way merge a la git, for use as unison merge tool

idiff3.sh

#! /bin/sh -Cefu

PATH=/bin:/usr/bin:/usr/local/bin
EDITOR=${VISUAL:-emacs}
# interactive merge a la git

# first, dump marked differences into a temporary file.
# ignore the exit status which just means conflicts have been found
t=$( mktemp idiff3.XXXXXXXXXXXX )
diff3 --label=local --label=old --label=remote -m $1 $2 $3 >> $t || true

# next, interactively edit the marked file
"${EDITOR}" $t

# hacky but simple: nonempty temp file means signed off
size=$( stat --printf='%s' $t )
case $size in
    (0)
        rm -f $t
        exit 3
        ;;
    (*)
        mv -i $t $4
        exit 0
        ;;
esac