git add -e: Explicitly specify that patch should have no color

After 4c7f1819 (make color.ui default to 'auto', 2013-06-10), the
patch file to be edited during 'git add -e' receives all the color
codes.  This is because diffopt.use_color defaults to -1, which
causes want_color to now return 'auto'.

By explicitly setting use_color to 0, we can ensure the diff output
has no color codes in it.

Signed-off-by: Andrew Wong <andrew.kw.w@gmail.com>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>