Execute pre-receive hook

This patch work in my project.
> Is this similar to what git does? It seems a bit odd to only report the error for the last ref.

Git return "pre-receive hook declined" for all refs :
https://github.com/git/git/blob/583b61c1af1bac12d6b48b2583ad508b5ab9964c/builtin/receive-pack.c#L801

I'll fix that in my patch.