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.
Ensure that setting the merge attribute forces the built-in default
`text` driver and does *not* honor the `merge.default` configuration
option. Further ensure that unsetting the merge attribute forces
a conflict (the `binary` driver).
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.
