All refs under `refs/pull` should only be changed from Gitea inside but
not by pushing from outside of Gitea.
This PR will prevent the pull refs update but allow other refs to be
updated on the same pushing with `--mirror` operations.
The main changes are to add checks on `update` hook but not
`pre-receive` because `update` will be invoked by every ref but
`pre-receive` will revert all changes once one ref update fails.
Lunny Xiao
ac34449747