ikiwiki hosting internals/ bugs/ git push too limited

`iki-git-hook-update` limits what pushes from external are allowed to do, which is necessary, but it may limit too much. It does not allow adding new branches, deleting branches; may not allow manipulating tags (need to check).

It makes sense to not allow any modification of the setup branch, and it should prevent outright deletion of the master branch. I think all other branch changes should be safe, and allowed. --Joey

> fixed. Note that for untrusted git push, ikiwiki's own checks will disallow changes to any branch other than master.