Previously, we called g_file_query_info() *again* on the source at the
very end of the copy.  This has the lame semantics that if the source
happened to be deleted, we would fail to apply attributes to the
destination.  This could even be a security flaw.

This commit changes things so that we query info from the source
*stream* after opening - i.e. on Unix we use the proper fstat() and
friends.  That way we operate more atomically.

https://bugzilla.gnome.org/show_bug.cgi?id=699959