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git: Support setup of the hooks directory at runtime

Patrick Steinhardt requested to merge pks-git-deprecate-ruby-hooks-directory into master

Gitaly's hooks are currently located in the Ruby directory, which we can (hopefully) get rid off soon. As a consequence, we also need to move our hooks somewhere else. While we could just create a different on-disk directory which is installed alongside with Gitaly, this creates more complexity than is worth it. Instead, we can just set up the hook directory at runtime by creating a temporary directory which has all the necessary hooks set up. Like this, we have one thing less that needs to be configured given that the only configuration we need for this to work is the location of the "gitaly-hooks" binary, which is easily derived from the binary path.

This new temporary directory is being created by the Git command factory at creation time. Ideally, this was done by just creating symlinking the required hooks to the gitaly-hooks binary. Unfortunately though, it depends on the first parameter being set to the name of the hook that is about to be executed. This is rather pointless: it could just derive the name from its zeroth argument, which would be set to the symlinks path anyway. That would require a separate change to gitaly-hooks though, and due to backwards-compatibility reasons that would require a full release cycle.

Instead, we're writing a wrapper script into the hook directory that's the same as our current gitlab-shell-hook script. There are two differences though:

1. The new script doesn't need the `GITALY_BIN_DIR` environment
   variable anymore to resolve the path of `gitaly-hooks`, but
   instead has its path resolved at creation-time of the script
   already.

2. The new script sets both the zeroth and first argument of
   `gitaly-hooks` to its own name. This will eventually allow us to
   migrate `gitaly-hooks` to resolve the hook name via its zeroth
   argument. In the end, this means that we can then get rid of the
   wrapper script altogether.

This change is done behind a new feature flag given that execution of hooks is essential to all writing code paths. This feature flag is injected with a random value into all of our tests to assert that they work correctly both with the flag enabled and disabled.

Changelog: changed

Closes #3932 (closed)

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