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>

Documentation/git-config.txt

@@ -177,6 +177,10 @@ Valid `<type>`'s include:
   ~/` from the command line to let your shell do the expansion.)
 - 'expiry-date': canonicalize by converting from a fixed or relative date-string
   to a timestamp. This specifier has no effect when setting the value.
+- 'color': When getting a value, canonicalize by converting to an ANSI color
+  escape sequence. When setting a value, a sanity-check is performed to ensure
+  that the given value is canonicalize-able as an ANSI color, but it is written
+  as-is.
 +
 
 --bool::
@@ -228,6 +232,8 @@ Valid `<type>`'s include:
 	output it as the ANSI color escape sequence to the standard
 	output.  The optional `default` parameter is used instead, if
 	there is no color configured for `name`.
++
+`--type=color [--default=<default>]` is preferred over `--get-color`.
 
 -e::
 --edit::

builtin/config.c

@@ -61,6 +61,7 @@ static int show_origin;
 #define TYPE_BOOL_OR_INT	3
 #define TYPE_PATH		4
 #define TYPE_EXPIRY_DATE	5
+#define TYPE_COLOR		6
 
 #define OPT_CALLBACK_VALUE(s, l, v, h, i) \
 	{ OPTION_CALLBACK, (s), (l), (v), NULL, (h), PARSE_OPT_NOARG | \
@@ -94,6 +95,8 @@ static int option_parse_type(const struct option *opt, const char *arg,
 			new_type = TYPE_PATH;
 		else if (!strcmp(arg, "expiry-date"))
 			new_type = TYPE_EXPIRY_DATE;
+		else if (!strcmp(arg, "color"))
+			new_type = TYPE_COLOR;
 		else
 			die(_("unrecognized --type argument, %s"), arg);
 	}
@@ -230,6 +233,11 @@ static int format_config(struct strbuf *buf, const char *key_, const char *value
 			if (git_config_expiry_date(&t, key_, value_) < 0)
 				return -1;
 			strbuf_addf(buf, "%"PRItime, t);
+		} else if (type == TYPE_COLOR) {
+			char v[COLOR_MAXLEN];
+			if (git_config_color(v, key_, value_) < 0)
+				return -1;
+			strbuf_addstr(buf, v);
 		} else if (value_) {
 			strbuf_addstr(buf, value_);
 		} else {
@@ -375,6 +383,20 @@ static char *normalize_value(const char *key, const char *value)
 		else
 			return xstrdup(v ? "true" : "false");
 	}
+	if (type == TYPE_COLOR) {
+		char v[COLOR_MAXLEN];
+		if (git_config_color(v, key, value))
+			die("cannot parse color '%s'", value);
+
+		/*
+		 * The contents of `v` now contain an ANSI escape
+		 * sequence, not suitable for including within a
+		 * configuration file. Treat the above as a
+		 * "sanity-check", and return the given value, which we
+		 * know is representable as valid color code.
+		 */
+		return xstrdup(value);
+	}
 
 	die("BUG: cannot normalize type %d", type);
 }

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"