completion: add completion for --submodule=* diff option

Teach git-completion.bash to complete --submodule= for git commands
which take diff options. Also teach completion for git-log to support
--diff-algorithms as well.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>