Add to stg-mdiff the ability to pass options to underlying diff opts.

Add to stg-mdiff the ability to pass options to underlying diff opts.

So we now have -O, similar to the existing option in stg diff, to pass
flags to git-diff when computing the 2 diffs to compare (-M, -C and -w
come to mind as being potentially useful here), and -o can be used to
tune the invocation of diff on those 2 diffs.

Note that -o is only there temporarily, and will disappear when a more
sophisticated process than plain diff will be used.

Signed-off-by: Yann Dirson <ydirson@altern.org>