Chained global hooks

Closes #32. Docs MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6721

See merge request !111

this fixes problem that tests succeeded locally but failed in ci
where parent dirs were missing

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.

Fetch repositories with --prune option by default.

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

Related issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/1277

I've reviewed the code for updating local and remote mirrors also  I've manually tested these features and all looks good.

Here is the description of the `--prune` option:


> --prune
>
> Before fetching, remove any remote-tracking references that no longer exist on the remote. Tags are not subject to pruning if they are fetched only because of the default tag auto-following or due to a --tags option. However, if tags are fetched due to an explicit refspec (either on the command line or in the remote configuration, for example if the remote was cloned with the --mirror option), then they are also subject to 


@brodock do you see any issues when fetching repos for GitLab Geo?

See merge request !110

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

Fix gitlab_custom_hook dependencies

See merge request !107

Add instrumentation to push hooks

See merge request !106

Related to #22053

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Use full repository path instead of extracting name

In order to implement nested groups https://gitlab.com/gitlab-org/gitlab-ce/issues/2772 we can not rely on old path with one slash that split namespace and project name like `namespace/project.git`. Now it can be both `namespace/project.git` and `namespace/namespace/namespace/project.git`. Because of that it makes no sense to extract part of full path. Instead we just pass path to API without change and let rails application do parsing/extraction. 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !102

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Git LFS remark in readme is no longer correct.

See merge request !101

Re-use the default logger for performance metrics

See #64 for the discussion leading up to this.

See merge request !100

This removes the need for configuring anything extra. To make scraping
easier each line is prefixed by "metrics:".

Fixes #64