fixing broken custom pre-receive calls

The gitlab pre-receive hook expects the call to exec to return 1
when a user has access. Doing an exit 0 effectively ended the
thread. No code would be executed after the call to exec.

This changes fixes the problem for the pre-receive hooks but you want
to check that GitlabAccess.exec is not used elsewhere. A quick
grep of the code seems to indicate this is not the case