When using difftool with vimdiff it can be unexpected that
the current directory changes to the root of the project.
Tell vim to chdir to the value of $GIT_PREFIX to fix this.
Care is taken to quote the variable so that vim expands it.
This avoids problems when directory names contain spaces.
Signed-off-by: David Aguilar <davvid@gmail.com> Reported-by: Frédéric Heitzmann <frederic.heitzmann@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>