file-undo-manager: handle undo_redo_flag internally

Nautilus undo manager reuses undo information based on a flag that marks whether
an undo / redo operation is currently being performed. Previously, the flag was
managed both internally and externally. This proved to be unnecessary and even
harmful, as it led to an unexpected behavior.

Remove push and pop functions and handle the flag explicitly, at the start and
at the end of an undo / redo operation. Replace external use of pop with the use
of a getter function, also introduced in this patch.

https://bugzilla.gnome.org/show_bug.cgi?id=759850