diff: factor out add_diff_options()

diff: factor out add_diff_options()

Add a function for appending the parseopts member of struct diff_options
to a struct option array.  Use it in two sites instead of accessing the
parseopts member directly.  Decoupling callers from diff internals like
that allows us to change the latter.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>