completion: add diff --color-moved[-ws]

completion: add diff --color-moved[-ws]

These options are available since git v2.15, but somehow
eluded from the completion script.

Note that while --color-moved-ws= accepts comma-separated
list of values, there is no (easy?) way to make it work
with completion (see e.g. [1]).

[1]: https://github.com/scop/bash-completion/issues/240

Acked-by: Matheus Tavares Bernardino <matheus.bernardino@usp.br>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>