Add a new configuration option, custom_hooks_dir. When this is set, we
will look for global custom hooks in:
    <custom_hooks_dir>/{pre-receive,update,post-receive}.d/*

When this is not set, default to <REPO_PATH>/hooks.

Add changelog entry

Editing changelog entry version

update hooks lookup to use <hook>.d/* from repository hooks dir

the order would be:

1. <repository>.git/custom_hooks/<hook_name> - per project hook
2. <repository>.git/custom_hooks/<hook_name>.d/* - per project hooks
3. <repository>.git/hooks/<hook_name>.d/* - global hooks

only executable files are matched and backup files excluded (*~)
and the resulting list is sorted per each lookup

This commit adds the option of having another set of global custom hooks
along with the already supported repository local custom hooks.

The repository local custom hook is executed first (if available). If
successful, execution continues with the global custom hook (if available).
This way, local custom hooks get priority over global custom hooks.

Global custom hooks can be enabled by placing an executable file into the
"custom_hooks" directory within gitlab-shell (create if it does not exist,
yet).

This commit takes the GitlabCustomHook a bit clother to the other hook
handling classes by receiving the repo_path as argument to initialize()
instead of passing it to each method.

This commit fixes an issue where an existing but unexecutable hook would
cause an uncaught execption.

This will allow us to not recreate branches that has been deleted on the upstream and the mirror.