ikiwiki hosting internals/ security/ push setup branch deny was broken

Ikisite created a broken symlink for the git update hook, so pushes to the setup branch were not denied. This could possibly lead to unauthorized code execution, although it would have to be paired with a site branch or backup/restore.

This is fixed, but any existing sites need to have their update hook manually fixed to link to /usr/bin/iki-git-hook-update

done --Joey