ref-filter: introduce match_atom_name()

Introduce match_atom_name() which helps in checking if a particular
atom is the atom we're looking for and if it has a value attached to
it or not.

Use it instead of starts_with() for checking the value of %(color:...)
atom. Write a test for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Thanks-to: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -189,6 +189,22 @@ static void pop_stack_element(struct ref_formatting_stack **stack)
 	*stack = prev;
 }
 
+static int match_atom_name(const char *name, const char *atom_name, const char **val)
+{
+	const char *body;
+
+	if (!skip_prefix(name, atom_name, &body))
+		return 0; /* doesn't even begin with "atom_name" */
+	if (!body[0]) {
+		*val = NULL; /* %(atom_name) and no customization */
+		return 1;
+	}
+	if (body[0] != ':')
+		return 0; /* "atom_namefoo" is not "atom_name" or "atom_name:..." */
+	*val = body + 1; /* "atom_name:val" */
+	return 1;
+}
+
 /*
  * In a format string, find the next occurrence of %(atom).
  */
@@ -687,6 +703,7 @@ static void populate_value(struct ref_array_item *ref)
 		int deref = 0;
 		const char *refname;
 		const char *formatp;
+		const char *valp;
 		struct branch *branch = NULL;
 
 		v->handler = append_atom;
@@ -721,10 +738,12 @@ static void populate_value(struct ref_array_item *ref)
 			refname = branch_get_push(branch, NULL);
 			if (!refname)
 				continue;
-		} else if (starts_with(name, "color:")) {
+		} else if (match_atom_name(name, "color", &valp)) {
 			char color[COLOR_MAXLEN] = "";
 
-			if (color_parse(name + 6, color) < 0)
+			if (!valp)
+				die(_("expected format: %%(color:<color>)"));
+			if (color_parse(valp, color) < 0)
 				die(_("unable to parse format"));
 			v->s = xstrdup(color);
 			continue;

t/t6302-for-each-ref-filter.sh

@@ -81,4 +81,8 @@ test_expect_success 'filtering with --contains' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(color) must fail' '
+	test_must_fail git for-each-ref --format="%(color)%(refname)"
+'
+
 test_done