After a failed boot, a Gitaly process can leave behind a forked process

Discovered via gitlab-development-kit#566 (comment 198901893)

The way to reproduce it there is:

• install GDK twice
• configure gitaly in both instances to use prometheus port 9236
• enable runit
• gdk start gitaly in both GDK's

Now you get a restart loop in one of them. Wait 5 minutes and you start seeing leaked Gitaly processes with PPID 1. Some of these processes use 100% CPU.