whichdiff.pl is a Perl script that finds the state.txt file to use
for minutely replication based on the highest node ID in your database.

Simply do a 

select max(id) from planet_osm_nodes

or if you're using flatnodes, osm2pgsql will report the highest node ID
in the flat storage when run. 

Then run whichdiff.pl with the node id as the only argument, and it will
give you the contents for your state.txt (by default it will use that 
state.txt file from the server which is two minutes before the one in 
which your latest node was generated).

Written by Frederik Ramm <frederik@remote.org>, public domain.