glocalfile: Only check for .hidden files if standard::is-hidden is requested

glocalfile: Only check for .hidden files if standard::is-hidden is requested

For OSTree, I use Gio and also really care about performance.  It's
disturbing to see open('.hidden') all over my straces and such.  At
the moment I have an explicit set of things to query, as opposed to
"standard::*", since even before this that also implies an lstat() of
the parent directory.

This matches up with what we do for all the other attributes.

See https://bugzilla.gnome.org/show_bug.cgi?id=587806
https://bugzilla.gnome.org/show_bug.cgi?id=691558