config: refactor management of color.ui's default value

The meaning of get_colorbool_found and get_diff_color_found is "the
config value if found, and -1 otherwise", but get_color_ui_found had a
slightly different meaning, as it has the value 0 (which corresponds to
the default value from the user point of view) when color.ui is unset.

Make get_color_ui_found default to -1, and make it explicit that 0 is the
default value when nothing else is found.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/config.c

@@ -329,6 +329,7 @@ static int get_colorbool(int print)
 {
 	get_colorbool_found = -1;
 	get_diff_color_found = -1;
+	get_color_ui_found = -1;
 	git_config_with_options(git_get_colorbool_config, NULL,
 				given_config_file, respect_includes);
 
@@ -339,6 +340,10 @@ static int get_colorbool(int print)
 			get_colorbool_found = get_color_ui_found;
 	}
 
+	if (get_colorbool_found < 0)
+		/* default value if none found in config */
+		get_colorbool_found = 0;
+
 	get_colorbool_found = want_color(get_colorbool_found);
 
 	if (print) {