stop assuming inf2cat is on the path; the signing code should call it
specifying path explicitly
add giturl option to override giturl in config file
remove reference repositories support; it isn't much use now that clones
do not need to be repeated on each interactive build
simplify git handling. You may specify branch or tag but not
both. There's some logging around the git operations and some checking code.
Merge pull request #34 from dickon/master

Stop assuming inf2cat is on the path since we shouldn't be using it that way. Allow git base URL to be specified in winbuild-prepare.ps1. Tidy up git repo handlign code.