The code initializing the merge driver registry accidentally
forgot a `goto done` in case of an error. Because of this the
next line, which registers the global shutdown callback for the
merge drivers, is only called when an error occured.
Fix this by adding the missing `goto done`. This fixes some
memory leaks when the global state is shut down.
Since the `apply` callback can defer, the `check` callback is not
necessary. Removing the `check` callback further makes the `payload`
unnecessary along with the `cleanup` callback.
Consumers can now register custom merged drivers with
`git_merge_driver_register`. This allows consumers to support the
merge drivers, as configured in `.gitattributes`. Consumers will be
asked to perform the file-level merge when a custom driver is
configured.
