Stop using gitlab-shell hooks -- but keep using gitlab-shell config

Part of "stage 1" of #1226 (comment 126519133)

We are trying to break the dependence of gitaly on gitlab-shell. This MR makes a big step in that direction: after it is merged we will no longer directly run code from gitlab-shell; we will run the hooks from a vendored copy of gitlab-shell instead.

We will still rely on a normal gitlab-shell installation on the Gitaly server, with a valid config file. Ultimately all configuration needed for the hooks to run should be in Gitaly's config.toml file but we are not doing that all at once.