gitaly-hooks: Fix deferred calls never running

While we do defer multiple calls in gitaly-hooks' main function, these aren't ever: the function never returns, but executes os.Exit(). While it's not much of a problem right now because ultimately we do not care for any of the deferred function calls, it invites for future errors in case somebody misses the fact that deferred calls never run.

Fix the issue by pulling out the logic into a separate run() function which has the usual return semantics.