t6006: drop "always" color config tests

We test the %C() format placeholders with a variety of
color-inducing options, including "--color" and
"-c color.ui=always". In preparation for the behavior of
"always" changing, we need to do something with those
"always" tests.

We can drop ones that expect "always" to turn on color even
to a file, as that will become a synonym for "auto", which
is already tested.

For the "--no-color" test, we need to make sure that color
would otherwise be shown. To do this, we can use
test_terminal, which enables colors in the default setup.

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

t/t6006-rev-list-format.sh

@@ -208,26 +208,11 @@ do
 		has_no_color actual
 	'
 
-	test_expect_success "$desc enables colors for color.diff" '
-		git -c color.diff=always log --format=$color -1 >actual &&
-		has_color actual
-	'
-
-	test_expect_success "$desc enables colors for color.ui" '
-		git -c color.ui=always log --format=$color -1 >actual &&
-		has_color actual
-	'
-
 	test_expect_success "$desc respects --color" '
 		git log --format=$color -1 --color >actual &&
 		has_color actual
 	'
 
-	test_expect_success "$desc respects --no-color" '
-		git -c color.ui=always log --format=$color -1 --no-color >actual &&
-		has_no_color actual
-	'
-
 	test_expect_success TTY "$desc respects --color=auto (stdout is tty)" '
 		test_terminal git log --format=$color -1 --color=auto >actual &&
 		has_color actual
@@ -240,6 +225,11 @@ do
 			has_no_color actual
 		)
 	'
+
+	test_expect_success TTY "$desc respects --no-color" '
+		test_terminal git log --format=$color -1 --no-color >actual &&
+		has_no_color actual
+	'
 done
 
 test_expect_success '%C(always,...) enables color even without tty' '