Use [g]vimdiff as the diff tool in git.

gitdiff.sh

#!/bin/sh

# shell wrapper to use gvimdiff and vimdiff as git differ.

# set up your ~/.gitconfig like this to use it:

#[diff]
#	external = gitdiff.sh
#
#[pager]
#	diff =
#	ndiff = less
#
#[alias]
#	ndiff = diff --no-ext-diff

# The ndiff alias is the "normal diff" in case you don't want to use vimdiff some times.

# This will use gvimdiff in a graphical environment (DISPLAY is set), or run an
# in-terminal vimdiff if not.

# Also, put this in your ~/.vimn/vimrc file to make exiting quicker.
# 
# if &diff
#    nmap ZZ :qall<CR>
# endif

if [ -n "$DISPLAY" ] ; then
	DIFF="/usr/local/bin/gvimdiff -RMmf"
else
	DIFF="/usr/local/bin/vimdiff -RMm"
fi

LEFT=${2}
RIGHT=${5}

exec $DIFF "$LEFT" "$RIGHT"