Add new --diff-opts/-O flag to diff- and status-related commands.

Add new --diff-opts/-O flag to diff- and status-related commands.

This new flag allows to pass arbitrary flags to the git-diff calls
underlying several StGIT commands.  It can be used to pass flags
affecting both diff- and status-generating commands (eg. -M or -C), or
flags only affecting diff-generating ones (eg. --color, --binary).

It supercedes --binary for commands where it was allowed,
'-O --binary' is now available for the same functionality.

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