This will allow us to prevent deadlocks when dealing with extremely
long outputs from `git fetch`. All the pipes (stdin, stdout, and
stderr) have limited buffers, and when one fills up, everything is
going to stop.

Workaround for a bug in the HTTPUNIX client.

1. Starting version 2.11, git changed the way the pre-receive flow works.

  - Previously, the new potential objects would be added to the main repo. If the
    pre-receive passes, the new objects stay in the repo but are linked up. If
    the pre-receive fails, the new objects stay orphaned in the repo, and are
    cleaned up during the next `git gc`.

  - In 2.11, the new potential objects are added to a temporary "alternate object
    directory", that git creates for this purpose. If the pre-receive passes, the
    objects from the alternate object directory are migrated to the main repo. If
    the pre-receive fails the alternate object directory is simply deleted.

2. In our workflow, the pre-recieve script calls the `/allowed` endpoint on the
   rails server. This `/allowed` endpoint calls out directly to git to perform
   various checks. These direct calls to git do _not_ have the necessary
   environment variables set which allow access to the "alternate object
   directory" (explained above). Therefore these calls to git are not able to
   access any of the new potential objects to be added during this push.

3. We fix this by passing the relevant environment variables
   (GIT_ALTERNATE_OBJECT_DIRECTORIES, GIT_OBJECT_DIRECTORY, and
   GIT_QUARANTINE_PATH) to the `/allowed` endpoint, which will then include
   these environment variables while calling out to git.

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.

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