po plugin(refreshpot): prevent PO references to be written

po plugin(refreshpot): prevent PO references to be written

... else we can fall into some kind of nasty infinite loop, when two ikiwiki
instances don't store their working copy of the repository at the same place:
every POT file update in one repository would trigger an update of the same POT
file in the others repository, and so on.

Signed-off-by: intrigeri <intrigeri@boum.org>