Add config_error_nonbool() helper function

This is used to report misconfigured configuration file that does not
give any value to a non-boolean variable, e.g.

	[section]
		var

It is perfectly fine to say it if the section.var is a boolean (it means
true), but if a variable expects a string value it should be flagged as
a configuration error.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1079,3 +1079,12 @@ int git_config_rename_section(const char *old_name, const char *new_name)
 	free(config_filename);
 	return ret;
 }
+
+/*
+ * Call this to report error for your variable that should not
+ * get a boolean value (i.e. "[my] var" means "true").
+ */
+int config_error_nonbool(const char *var)
+{
+	return error("Missing value for '%s'", var);
+}