Use lower case option name in eclcc --defaultgitprefix

It isn't possible to pass mixed case options to eclccserver via the debug options.  That means the following command does not work:

ecl run hthor --main demo.main@ghalliday/gch-ecldemo-b.git#version3 --server=192.168.86.202:8010 -f--fetchrepos -
f--defaultGitPrefix=git://github.com

so switch to --defaultgitprefix instead.