ttlogmerge: A new helper script for the Unison file synchroniser

A helper script for the Unison file synchroniser. One option for Unison is to use an external program to merge two files which have been change in both roots. This script uses this feature to merge two TagTime log files which have been created on different machines.

In principle, this allows you to run TagTime on multiple computers and, as long as they have the same period and random seeds, create a merged file which tracks what you were doing on all your machines.

(Inspired by https://groups.google.com/forum/#!topic/tagtime/Etey8bFaYSk)