With minimum settings.

Set the following lines in ~/.gitconfig.

[diff]

[difftool "bcompare"]

        cmd = bcompare "$LOCAL" "$REMOTE"

Prepare the following shell script git-bcompare in a $PATH'ed directory.  It's from Seba's blog post and modified a little.

#!/bin/sh

git diff --name-only "$@" | while read filename; do

    git difftool "$@" --no-prompt -t bcompare "$filename" &

done

Now, a command "git bcompare" is available.