t4026: test "normal" color

t4026: test "normal" color

If the user specifiers "normal" for a foreground color, this
should be a noop (while this may sound useless, it is the
only way to specify an unchanged foreground color followed
by a specific background color).

We also check that color "-1" does the same thing. This is
not documented, but has worked forever, so let's make sure
we keep supporting it.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>