ref-filter: introduce color_atom_parser()

Introduce color_atom_parser() which will parse a "color" atom and
store its color in the "used_atom" structure for further usage in
populate_value().

Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Karthik Nayak <Karthik.188@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -29,10 +29,21 @@ typedef enum { FIELD_STR, FIELD_ULONG, FIELD_TIME } cmp_type;
 static struct used_atom {
 	const char *name;
 	cmp_type type;
+	union {
+		char color[COLOR_MAXLEN];
+	} u;
 } *used_atom;
 static int used_atom_cnt, need_tagged, need_symref;
 static int need_color_reset_at_eol;
 
+static void color_atom_parser(struct used_atom *atom, const char *color_value)
+{
+	if (!color_value)
+		die(_("expected format: %%(color:<color>)"));
+	if (color_parse(color_value, atom->u.color) < 0)
+		die(_("unrecognized color: %%(color:%s)"), color_value);
+}
+
 static struct {
 	const char *name;
 	cmp_type cmp_type;
@@ -70,7 +81,7 @@ static struct {
 	{ "symref" },
 	{ "flag" },
 	{ "HEAD" },
-	{ "color" },
+	{ "color", FIELD_STR, color_atom_parser },
 	{ "align" },
 	{ "end" },
 };
@@ -158,6 +169,7 @@ int parse_ref_filter_atom(const char *atom, const char *ep)
 	used_atom[at].type = valid_atom[i].cmp_type;
 	if (arg)
 		arg = used_atom[at].name + (arg - atom) + 1;
+	memset(&used_atom[at].u, 0, sizeof(used_atom[at].u));
 	if (valid_atom[i].parser)
 		valid_atom[i].parser(&used_atom[at], arg);
 	if (*atom == '*')
@@ -816,6 +828,7 @@ static void populate_value(struct ref_array_item *ref)
 
 	/* Fill in specials first */
 	for (i = 0; i < used_atom_cnt; i++) {
+		struct used_atom *atom = &used_atom[i];
 		const char *name = used_atom[i].name;
 		struct atom_value *v = &ref->value[i];
 		int deref = 0;
@@ -856,14 +869,8 @@ static void populate_value(struct ref_array_item *ref)
 			refname = branch_get_push(branch, NULL);
 			if (!refname)
 				continue;
-		} else if (match_atom_name(name, "color", &valp)) {
-			char color[COLOR_MAXLEN] = "";
-
-			if (!valp)
-				die(_("expected format: %%(color:<color>)"));
-			if (color_parse(valp, color) < 0)
-				die(_("unable to parse format"));
-			v->s = xstrdup(color);
+		} else if (starts_with(name, "color:")) {
+			v->s = atom->u.color;
 			continue;
 		} else if (!strcmp(name, "flag")) {
 			char buf[256], *cp = buf;