Item11267: rewrite trunk.foswiki.org update script

Still need to decide what to do with Foswiki.pm version string.

Also rather than using _allFoswiki master branch, probably makes sense
to use a new local branch with additional submodules.

git-svn-id: http://svn.foswiki.org/trunk/core@17864 0b4bb1d4-4e5a-0410-9cc4-b2b747904278