add-interactive: retain colorbool values longer

add-interactive: retain colorbool values longer

Most of the diff code stores the decision about whether to show color as
a git_colorbool, and evaluates it at point-of-use with want_color().
This timing is important for reasons explained in daa0c3d971 (color:
delay auto-color decision until point of use, 2011-08-17).

The add-interactive code instead converts immediately to strict boolean
values using want_color(), and then evaluates those. This isn't wrong.
Even though we pass the bool values to diff_use_color(), which expects a
colorbool, the values are compatible. But it is unlike the rest of the
color code, and is questionable from a type-system perspective (but C's
typing between enums, ints, and bools is weak enough that the compiler
does not complain).

Let's switch it to the more usual way of calling want_color() at the
point of use.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>