Fix default lookup of global custom hooks

In !1052 (merged) we stopped creating hooks symlinks for new repositories. These symlinks were pointing to the old gitlab-shell hooks, which we were ignoring anyway.

However, the "global custom hooks" feature was relying on the symlink to find the /path/to/gitlab-shell/hooks directory.

This MR changes the lookup of the default value to use gitlab-shell's ROOT_PATH constant, which still points to /path/to/gitlab-shell.

Fixes #1758 (closed)