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

t/t4026-color.sh

@@ -53,6 +53,14 @@ test_expect_success '256 colors' '
 	color "254 bold 255" "[1;38;5;254;48;5;255m"
 '
 
+test_expect_success '"normal" yields no color at all"' '
+	color "normal black" "[40m"
+'
+
+test_expect_success '-1 is a synonym for "normal"' '
+	color "-1 black" "[40m"
+'
+
 test_expect_success 'color too small' '
 	invalid_color "-2"
 '