builtin/config: introduce color type specifier

As of this commit, the canonical way to retreive an ANSI-compatible
color escape sequence from a configuration file is with the
`--get-color` action.

This is to allow Git to "fall back" on a default value for the color
should the given section not exist in the specified configuration(s).

With the addition of `--default`, this is no longer needed since:

  $ git config --default red --type=color core.section

will be have exactly as:

  $ git config --get-color core.section red

For consistency, let's introduce `--type=color` and encourage its use
with `--default` together over `--get-color` alone.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1300-repo-config.sh

@@ -931,6 +931,36 @@ test_expect_success 'get --expiry-date' '
 	test_must_fail git config --expiry-date date.invalid1
 '
 
+test_expect_success 'get --type=color' '
+	rm .git/config &&
+	git config foo.color "red" &&
+	git config --get --type=color foo.color >actual.raw &&
+	test_decode_color <actual.raw >actual &&
+	echo "<RED>" >expect &&
+	test_cmp expect actual
+'
+
+cat >expect << EOF
+[foo]
+	color = red
+EOF
+
+test_expect_success 'set --type=color' '
+	rm .git/config &&
+	git config --type=color foo.color "red" &&
+	test_cmp expect .git/config
+'
+
+test_expect_success 'get --type=color barfs on non-color' '
+	echo "[foo]bar=not-a-color" >.git/config &&
+	test_must_fail git config --get --type=color foo.bar
+'
+
+test_expect_success 'set --type=color barfs on non-color' '
+	test_must_fail git config --type=color foo.color "not-a-color" 2>error &&
+	test_i18ngrep "cannot parse color" error
+'
+
 cat > expect << EOF
 [quote]
 	leading = " test"