diff: Introduce --diff-algorithm command line option

diff: Introduce --diff-algorithm command line option

Since command line options have higher priority than config file
variables and taking previous commit into account, we need a way
how to specify myers algorithm on command line. However,
inventing `--myers` is not the right answer. We need far more
general option, and that is `--diff-algorithm`.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>